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 factor1,2. Major research objectives are the identification of endogenous activators and the determination of cellular and tissue functions of these novel channels. Here we show activation of TRPC5 homomultimeric and TRPC5-TRPC1 heteromultimeric channels3-5 by extracellular reduced thioredoxin acting 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 unknown6-9. Particularly high extracellular concentrations of thioredoxin are apparent in rheumatoid arthritis8,10-12, an inflammatory joint disease disabling millions of people worldwide13. We show that TRPC5 and TRPC1 are expressed in secretory fibroblast-like synoviocytes from patients with rheumatoid arthritis, endogenous TRPC5-TRPC1 channels of the cells are activated by reduced thioredoxin, and blockade of the channels enhances secretory activity and prevents suppression of secretion by thioredoxin. The data suggest a novel ion channel activation mechanism that couples extracellular thioredoxin to cell function.Striking activators of TRPC5 are extracellular lanthanide ions4,14,15. Effects of these ions depend on a glutamic acid residue at position 54314 in the predicted extracellular loop adjacent to the ion pore (Supplementary Fig. 1-2). This structural feature may, therefore, have functional importance in enabling extracellular factors to activate the channels. Because lanthanides are unlikely 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 channels1-5. TRPC5 and TRPC4 have similar functional properties4 and both form heteromultimers with TRPC13-5, a subunit that has weak targeting to the plasma membrane when expressed in isolation3,16. 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 TRPC515,16. There was channel activation with the characteristic current-voltage relationship (I-V) of TRPC5 and block by 2-APB, an inhibitor of TRPC55 (Fig. 1a, b, d). Current recovered on wash-out of DTT (data not shown). Similarly, the membrane-impermeable disulphide reducing agent TCEP (Fig. 1c, d) activated TRPC5, whereas the thiol reagent MTSET had no effect (Fig. 1d). TRPC5 was inhibited by cadmium ions only after pre-treatment with DTT ( Fig. 1e, f), consistent with the metal ion acting by re-engaging cysteines17. Other TRP channels lacking the cysteine pair in a similar po...

Highlights The G clade 23403A>G mutation on the spike glycoprotein (S-protein) encodes a virulent strain of SARS-CoV-2. D614 G mutation causes a loss of H-bond between loop (Chain A) and the α-helix (Chain B) results in a more flexible loop region. A more dynamic structure made the S-protein RRAR binding site more accessible from furin cleavage. SARS-CoV-2 strain being more accessible for cleavage, enhances the viral entry to the host cell.

Objective-To determine whether calcium-permeable channels are targets for the oxidized phospholipids: 1-palmitoyl-2-glutaroyl-phosphatidylcholine (PGPC) and 1-palmitoyl-2-oxovaleroyl-phosphatidylcholine (POVPC). Methods and Results-Oxidized phospholipids are key factors in inflammation and associated diseases, including atherosclerosis; however, the initial reception mechanisms for cellular responses to the factors are poorly understood. Low micromolar concentrations of PGPC and POVPC evoked increases in intracellular calcium in human embryonic kidney 293 cells that overexpressed human transient receptor potential canonical 5 (TRPC5) but not human TRP melastatin (TRPM) 2 or 3. The results of electrophysiological experiments confirmed stimulation of TRPC5. To investigate relevance to endogenous channels, we studied proliferating vascular smooth muscle cells from patients undergoing coronary artery bypass surgery. PGPC and POVPC elicited calcium entry that was inhibited by anti-TRPC5 or anti-TRPC1 antibodies or dominant-negative mutant TRPC5. Calcium release did not occur. The effect was functionally relevant because it enhanced cell migration. The actions of PGPC and POVPC depended on G i/o proteins but not on previously identified G protein-coupled receptors for oxidized phospholipids. Conclusion-Stimulation of calcium-permeable TRPC5-containing channels may be an early event in cellular responses to oxidized phospholipids that couples to cell migration and requires an unidentified G protein-coupled receptor.

TRPC channels are a subset of the transient receptor potential (TRP) proteins widely expressed in mammalian cells. They are thought to be primarily involved in determining calcium or sodium entry and have broad-ranging functions that include regulation of cell proliferation, motility and contraction. The channels do not respond to a single stimulator but rather are activated or modulated by a multiplicity of factors, potentially existing as integrators at the plasma membrane. This review considers the sensitivity of TRPCs to lipid factors, with focus on sensitivities to diacylglycerols, lysophospholipids, arachidonic acid and its metabolites, sphingosine-1-phosphate (S1P), cholesterol and derivatives, and other lipid factors such as gangliosides. Promiscuous and selective lipid-sensing are apparent. In many cases the lipids stimulate channel function or increase insertion of channels in the membrane. Both direct and indirect (receptor-dependent) lipid effects are evident. Although information is limited, the lipid profiles are consistent with TRPCs having close working relationships with phospholipase C and A2 enzymes. We need much more information about lipid-sensing by TRPCs if we are to fully appreciate its significance, but the available data suggest that lipid-sensing is a key, but not exclusive, aspect of TRPC biology.