CALHM1 is an Extracellular Ca2+- and Voltage-Gated ATP Permeable Ion Channel

Abstract

We identified CALHM1 as a pore-forming subunit of a plasma membrane ion channel with weak ion selectivity and unique coupled allosteric gating regulation by voltage and extracellular Ca2+ (Ca2+o) (PNAS 109: E1963 (2012)). CALHM1 is expressed in mouse cortical neurons where it accounts for low [Ca2+]o-enhanced conductance and action potential firing. We recently determined that a CALHM1 channel is a hexamer with an estimated effective pore diameter ∼14A.Extracellular adenosine 5'-triphosphate (ATP) plays critical roles in physiological and signal transduction processes. We examined whether ATP can permeate CALHM1 channels. Reducing [Ca2+]o to activate CALHM1 induced ATP release from hCALHM1-expressing HeLa and COS-1 cells, and Xenopus oocytes. Neither CALHM1 expression nor lowering [Ca2+]o caused cell damage. Involvement of other possible mechanisms was ruled out because ATP release was unaffected by Brefeldin A (vesicular release), DCPIB (volume-sensitive Cl− channels), A438079 (P2X7 receptors), heptanol and carbenoxolone (connexins and pannexins). In contrast, ruthenium red (RuR), which inhibits CALHM1 currents, abolished low [Ca2+]o-evoked ATP release. Thus, CALHM1 expression induces a novel ATP permeability. Ca2+o inhibited ATP release with IC50 = 495 μM and Hill coefficient of 1.9, kinetic properties similar to those of its gating regulation. Membrane depolarization activates CALHM1 channels in normal [Ca2+]o. hCALHM1-expressing but not mock-transfected cells released ATP in response to high [K+]o-induced depolarization in normal [Ca2+]o, which was inhibited by RuR but not by connexin and pannexin-1 blockers. Thus, regulation of ATP release is correlated with the gating properties of CALHM1 channels, indicating that the CALHM1 channel is the conduit for ATP release. These results demonstrate that CALHM1 is a voltage-gated ATP release channel that may contribute to ATP release in physiological and pathological conditions.