Protection of TRPC7 cation channels from calcium inhibition by closely associated SERCA pumps.

Numerous studies have demonstrated that members of the transient receptor potential (TRP) superfamily of channels are involved in regulated Ca2+ entry. Additionally, most Ca2+-permeable channels are themselves regulated by Ca2+, often in complex ways. In the current study, we have investigated the regulation of TRPC7, a channel known to be potentially activated by both store-operated mechanisms and non-store-operated mechanisms involving diacylglycerols. Surprisingly, we found that activation of TRPC7 channels by diacylglycerol was blocked by the SERCA pump inhibitor thapsigargin. The structurally related channel, TRPC3, was similarly inhibited. This effect depended on extracellular calcium and on the driving force for Ca2+ entry. The inhibition is not due to calcium entry through store-operated channels but rather results from calcium entry through TRPC7 channels themselves. The effect of thapsigargin was prevented by inhibition of calmodulin and was mimicked by pharmacological disruption of the actin cytoskeleton. Our results suggest the presence of a novel mechanism involving negative regulation of TRPC channels by calcium entering through the channels. Under physiological conditions, this negative feedback by calcium is attenuated by the presence of closely associated SERCA pumps.

The role of canonical transient receptor potential 7 in B-cell receptor-activated channels.

Phospholipase C signaling stimulates Ca2+ entry across the plasma membrane through multiple mechanisms. Ca2+ store depletion stimulates store-operated Ca2+-selective channels, or alternatively, other phospholipase C-dependent events activate Ca2+-permeable non-selective cation channels. Transient receptor potential 7 (TRPC7) is a non-selective cation channel that can be activated by both mechanisms when ectopically expressed, but the regulation of native TRPC7 channels is not known. We knocked out TRPC7 in DT40 B-cells, which expresses both forms of Ca2+ entry. No difference in the store-operated current I(crac) was detected between TRPC7-/- and wild-type cells. Wild-type cells demonstrated nonstore-operated cation entry and currents in response to activation of the B-cell receptor or protease-activated receptor 2, intracellular dialysis with GTPgammaS, or application of the synthetic diacylglycerol oleyl-acetyl-glycerol. These responses were absent in TRPC7-/- cells but could be restored by transfection with human TRPC7. In conclusion, in B-lymphocytes, TRPC7 appeared to participate in the formation of ion channels that could be activated by phospholipase C-linked receptors. This represents the first demonstration of a physiological function for endogenous TRPC7 channels.

Mechanism of inhibition of TRPC cation channels by 2-aminoethoxydiphenylborane.

We investigated the actions of the organoborane, 2-aminoethoxydiphenylborane (2APB), on Ca2+ signaling in wild-type human embryonic kidney (HEK) 293 cells and in HEK293 cells stably expressing canonical transient receptor potential (TRPC) channels. Previous reports have suggested that 2APB inhibits agonist activation of TRPC channels because of its ability to act as a membrane-permeant inhibitor of inositol 1,4,5-trisphosphate (IP3) receptors. 2APB was specifically said to inhibit TRPC3 channels when activated through a phospholipase C-linked receptor but not when activated more directly by a synthetic diacylglycerol, oleyl-acetyl-glycerol (OAG) [Science (Wash DC) 287:1647-1651, 2000]. However, we subsequently reported that IP3 does not activate TRPC3; rather the mechanism of activation by phospholipase C-linked receptors seemed to result from diacylglycerol [J Biol Chem 278:16244-16252, 2003]. Thus, the current study was carried out to address the mechanism of action of 2APB in inhibiting TRPC channels. We found that, although the release of Ca2+ by a muscarinic agonist was reduced by high concentrations of 2APB, this effect was indistinguishable from that seen when stores were discharged by thapsigargin, which does not involve IP3 receptors. This indicates that 2APB is incapable of significant inhibition of IP3 receptors when applied to intact cells. We found that 2APB partially inhibits divalent cation entry in cells expressing TRPC3, TRPC6, or TRPC7 and that this partial inhibition was observed whether the channels were activated by a muscarinic agonist or by OAG. Thus, as concluded for store-operated channels, 2APB seems to inhibit TRPC channels by a direct mechanism not involving IP3 receptors.

Canonical transient receptor potential TRPC7 can function as both a receptor- and store-operated channel in HEK-293 cells.

Previous studies on the activation mechanism of canonical transient receptor potential (TRPC) channels have often produced conflicting conclusions. All seven have been shown to be activated by phospholipase C (PLC)-coupled receptors, but TRPC1, TRPC2, TRPC3, TRPC4, TRPC5, and TRPC7 have also been proposed to function as store-operated channels.(1)1Although PLC activation inevitably leads to activation of store-operated channels, in this report when we refer to PLC-activated channels, we mean those channels that are specifically activated by PLC independently of store depletion. In the case of TRPC3, the expression environment and the expression level appear to determine the mode of regulation. Evidence of a close structural relative of TRPC3, TRPC7, has been presented that this channel is activated by receptor activation or by store depletion. On the basis of previous findings for TRPC3, we reasoned that subtle differences in structure or expression conditions might account for the apparent distinct gating mechanisms of TRPC7. To reexamine the mode of activation of TRPC7, we stably and transiently transfected human embryonic kidney (HEK)-293 cells with cDNA encoding for human TRPC7. We examined the ability of a PLC-activating agonist and an intracellular Ca(2+) store-depleting agent to activate these channels. Our findings demonstrate that when transiently expressed in HEK-293 cells, TRPC7 forms channels that are activated by PLC-stimulating agonists, but not by Ca(2+) store depletion. However, when stably expressed in HEK-293 cells, TRPC7 can be activated by either Ca(2+) store depletion or PLC activation. To our knowledge, this is the first demonstration of a channel protein that can be activated by both receptor- and store-operated modes in the same cell. In addition, the results reconcile the apparently conflicting findings of other laboratories regarding TRPC7 regulation.

Mechanisms of phospholipase C-regulated calcium entry.

In a variety of cell types, activation of phospholipase C-linked receptors results in the generation of intracellular Ca2+ signals comprised of components of both intracellular Ca2+ release, and enhanced entry of Ca2+ across the plasma membrane. This entry of Ca2+ occurs by either of two general mechanisms: the release of stored Ca2+ can activate, by an unknown mechanism, store-operated channels in the plasma membrane, a process known as capacitative calcium entry. Alternatively, second messengers generated at the plasma membrane can activate Ca2+ channels more directly, a non-capacitative calcium entry process. This review summarizes current knowledge of the underlying signaling mechanisms and the nature of the channel molecules responsible for these two general categories of regulated Ca2+ entry.