Functional characterization of recombinant human CIC-4 chloride channels in cultured mammalian cells

Members of the ClC chloride channel family participate in several physiological processes and are linked to human genetic diseases. The physiological role of ClC-4 is unknown and previous detailed characterizations of recombinant human ClC-4 (hClC-4) have provided conflicting results. To reexamine the hClC-4 phenotype, recombinant hClC-4 was expressed in three distinct mammalian cell lines and characterized using patch-clamp techniques. In all cells, the expression of hClC-4 generated strongly outward-rectifying Cl^- currents with the conductance sequence: SCN^- ≫ NO₃^- ≫ Cl^- > Br^- ≈ I^- ≫ aspartate. Continuous activity of hClC-4 was sustained to different degrees by internal nucleotides: ATP ≈ ATPγS ≫ AMP-PNP ≈ GTP > ADP. Although non-hydrolysable nucleotides are sufficient for channel function, ATP hydrolysis is required for full activity. Changing the extracellular (2 mM or nominal Ca^a+-free) or intracellular Ca²⁺ (25 or 250 nM) concentration did not alter hClC-4 currents. Acidification of external pH (pH_o) inhibited hClC-4 currents (half-maximal inhibition ≈ 6.19), whereas neither external alkalinization to pH 8.4 nor internal acidification to pH 6.0 reduced current levels. Single-channel recordings demonstrated a Cl^- channel active only at depolarizing potentials with a slope conductance of ∼3 pS. Acidic pH_o did not alter single-channel conductance. We conclude that recombinant hClC-4 encodes a small-conductance, nucleotide-dependent, Ca²⁺ -independent outward-rectifying chloride channel that is inhibited by external acidification. This detailed characterization will be highly valuable in comparisons of hClC-4 function with native chloride channel activities and for future structure-function correlations.