TRPC channel activation by extracellular thioredoxin

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From: Nature(Vol. 451, Issue 7174)
Publisher: Nature Publishing Group
Document Type: Article
Length: 3,858 words
Lexile Measure: 1440L

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Mammalian homologues of Drosophila melanogaster transient receptor potential (TRP) are a large family of multimeric cation channels that act, or putatively act, as sensors of one or more chemical factor (1,2). Major research objectives are the identification of endogenous activators and the determination of cellular and tissue functions of these channels. Here we show the activation of TRPC5 (canonical TRP 5) homomultimeric and TRPC5-TRPCI heteromultimeric channels (3-5) by extracellular reduced thioredoxin, which acts by breaking a disulphide bridge in the predicted extracellular loop adjacent to the ion-selectivity filter of TRPC5. Thioredoxin is an endogenous redox protein with established intracellular functions, but it is also secreted and its extracellular targets are largely unknown (6-9). Particularly high extracellular concentrations of Thoredoxin are apparent in rheumatoid arthritis (8,10-12), an inflammatory joint disease that disables millions of people worldwide (13). We show that TRPC5 and TRPC1 are expressed in secretory fibroblast-like synoviocytes from patients with rheumatoid arthritis, that endogenous TRPC5-TRPCI channels of the cells are activated by reduced thioredoxin, and that blockade of the channels enhances secretory activity and prevents the suppression of secretion by thioredoxin. The data indicate the presence of a previously unrecognized ion-channel activation mechanism that couples extracellular thioredoxin to cell function.

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TRPC5 is markedly activated by extracellular lanthanide ions (4,14,15) The effects of these ions depend on a glutamic acid residue at position 543 (ref. 14) in the predicted extracellular loop adjacent to the ion pore (Supplementary Figs 1 and 2). This structural feature may therefore have functional importance in enabling extracellular factors to activate the channels. Because lanthanides are unlikely to be physiological activators, we were interested in alternatives and developed a hypothesis based on amino acid sequence alignment, which showed two cysteine residues near glutamic acid 543 that are conserved in TRPC5, TRPC4 and TRPC1 (Supplementary Fig. 2), a subset of the seven TRPC channels (1-5). TRPC5 and TRPC4 have similar functional properties' and both form heteromultimers with TRPC1 (refs 3-5), a subunit that has weak targeting to the plasma membrane when expressed in isolation (3,16).

Pairs of cysteine residues may be covalently linked by a disulphide bridge that can be cleaved by reduction. We therefore applied the chemical reducing agent dithiothreitol (DTT) to HEK-293 cells expressing TRPC5 (refs 15, 16). There was channel activation with the characteristic Current-voltage (I-V) relationship of TRPC5 and blocking by 2-aminoethoxydiphenyl borate (2-APB), an inhibitor of TRPC5 (ref. 5) (Fig. la, b, d). Current recovered on wash-out of DTT (data not shown). Similarly, the membrane-impermeable disulphide reducing agent Tris (2-carboxyethyl) phosphine hydrochloride (TCEP; Fig. lc, d) activated TRPC5, whereas the thiol reagent [2-(trimethylammonium) ethyl] methanethiosulphonate bromide (MTSET) had no effect (Fig. ld). TRPC5 was inhibitod by cadmium ions only after pretreatment with DTT (Fig. le, f), which is consistent with the metal ions acting by re-engaging cysteine residues (17). Other TRP channels lacking the cysteine pair in a similar position were unresponsive to DTT (Supplementary Figs 2 and 3). The data support the hypothesis that the cysteine pair in TRPC5 normally engages...

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Gale Document Number: GALE|A189749325