Lanthanides potentiate TRPC5 currents by an action at extracellular sites close to the pore mouth

Mammalian members of the classical transient receptor potential channel (TRPC) subfamily (TRPC1-7) are Ca2+-permeable cation channels involved in receptor-mediated increases in intracellular Ca2+. Unlike most other TRP-related channels, which are inhibited by La3+ and Gd3+, currents through TRPC4 and TRPC5 are potentiated by La3+. Because these differential effects of lanthanides on TRPC subtypes may be useful for clarifying the role of different TRPCs in native tissues, we characterized the potentiating effect in detail and localized the molecular determinants of potentiation by mutagenesis. Whole cell currents through TRPC5 were reversibly potentiated by micromolar concentrations of La3+ or Gd3+, whereas millimolar concentrations were inhibitory. By comparison, TRPC6 was blocked to a similar extent by La3+ or Gd3+ at micromolar concentrations and showed no potentiation. Dual effects of lanthanides on TRPC5 were also observed in outside-out patches. Even at micromolar concentrations, the single channel conductance was reduced by La3+, but reduction in conductance was accompanied by a dramatic increase in channel open probability, leading to larger integral currents. Neutralization of the negatively charged amino acids Glu(543) and Glu(595)/Glu(598), situated close to the extracellular mouth of the channel pore, resulted in a loss of potentiation, and, for Glu(595)/Glu(598) in a modification of channel inhibition. We conclude that in the micromolar range, the lanthanide ions La3+ and Gd3+ have opposite effects on whole cell currents through TRPC5 and TRPC6 channels. The potentiation of TRPC4 and TRPC5 by micromolar La3+ at extracellular sites close to the pore mouth is a promising tool for identifying the involvement of these isoforms in receptor-operated cation conductances of native cells.