A novel capacitative calcium entry channel expressed in excitable cells

In addition to voltage‐gated calcium influx, capacitative calcium entry (CCE) represents a major pathway for calcium entry into the cell. Here we report the structure, expression and functional properties of a novel CCE channel, TRP5. This channel is a member of a new subfamily of mammalian homologues of the Drosophila transient receptor potential (TRP) protein, now comprising TRP5 (also CCE2) and the structurally related CCE1 (also TRP4). Like TRP4, TRP5 forms ion channels mainly permeable for Ca 2+ which are not active under resting conditions but can be activated by manoeuvres known to deplete intracellular calcium stores. Accordingly, dialysis of TRP5‐expressing cells with inositol‐(1,4,5)‐trisphosphate evokes inward rectifying currents which reversed polarity at potentials more positive than +30 mV. Ca 2+ store depletion with thapsigargin induced TRP5‐mediated calcium entry dependent on the concentration of extracellular calcium, as seen by dual wavelength fura‐2 fluorescence ratio measurements. TRP5 transcripts are expressed almost exclusively in brain, where they are present in mitral cells of the olfactory bulb, in lateral cerebellar nuclei and, together with TRP4 transcripts, in CA1 pyramidal neurons of the hippocampus, indicating the presence of CCE channels in excitable cells and their participation in neuronal calcium homeostasis.