Permeation through store-operated CRAC channels in divalent-free solution: potential problems and implications for...

CRAC channels are key calcium conduits in both physiological and pathological states. Understanding how these channels are controlled is important as this will not only provide insight into a novel signal transduction pathway coupling intracellular stores to the channels in the plasma membrane, but might also be of clinical relevance. Determining the molecular identity of the CRAC channels will certainly be a major step forward. Like all Ca(2+)-selective channels, CRAC channels lose their selectivity in divalent-free external solution to support large monovalent Na(+) currents. This approach has provided new insight into channel permeation and selectivity, and identifies some interesting differences between CRAC channels and voltage-operated calcium channels (VOCCs). Studies in divalent-free solution are a double-edged sword, however. Electrophysiologists need to be wary because some of the conditions used to study I(CRAC) in divalent-free external solution, notably omission of Mg(2+)/Mg-ATP from the recording pipette solution, activates an additional current permeating through Mg(2+)-nucleotide-regulated metal ion current (MagNuM; TRPM7) channels. This channel underlies the large single-channel events that have been attributed to CRAC channels in the past and which have been used to as a tool to identify store-operated channels in native cells and recombinant expression systems.Are we any closer to identifying the elusive CRAC channel gene(s)? TRPV6 seemed a very attractive candidate, but one of the main arguments supporting it was a single-channel conductance in divalent-free solution similar to that for CRAC reported under conditions where MagNuM is active. We now know that the conductance of TRPV6 is approximately 200-fold larger than that of CRAC in native tissue. Moreover, it is unclear if TRPV6 is store-operated. Further work on TRPV6, particularly whether its single-channel conductance is still high under conditions where it apparently forms multimers with endogenous store-operated channels, and whether it is activated by a variety of store depletion protocols, will be helpful in finally resolving this issue.