Extracellular Chloride Modulates the Desensitization Kinetics of Acid-sensing Ion Channel 1a (ASIC1a)

Acid-sensing ion channels (ASICs) are sodium channels gated by extracellular protons. The recent crystallization of ASIC1a identified potential binding sites for Cl- in the extracellular domain that are highly conserved between ASIC isoforms. However, the significance of Cl- binding is unknown. We investigated the effect of Cl- substitution on heterologously expressed ASIC1a current and H+-gated currents from hippocampal neurons recorded by whole-cell patch clamp. Replacement of extracellular Cl- with the impermeable and inert anion methanesulfonate (MeSO3-) caused ASIC1a currents to desensitize at a faster rate and attenuated tachyphylaxis. However, peak current amplitude, pH sensitivity, and selectivity were unchanged. Other anions, including Br-, I-, and thiocyanate, also altered the kinetics of desensitization and tachyphylaxis. Mutation of the residues that form the Cl--binding site in ASIC1a abolished the modulatory effects of anions. The results of anion substitution on native ASIC channels in hippocampal neurons mirrored those in heterologously expressed ASIC1a and altered acid-induced neuronal death. Anion modulation of ASICs provides new insight into channel gating and may prove important in pathological brain conditions associated with changes in pH and Cl-.