Bidirectional sensitivity of CALHM1 channel to protons from both sides of plasma membrane

Calcium homeostasis modulator 1 (CALHM1), a newly discovered voltage-dependent nonselective ion channel, has drawn attention for its role in neuronal activity and taste sensation. Its sluggish voltage-dependent activation is facilitated by lowering extracellular Ca2+ concentration ([Ca2+]e). Here, we investigated the effects of extracellular and intracellular pH (pHe and pHi) on human CALHM1. When normalized to the amplitude of the CALHM1 current (ICALHM1) under whole cell patch clamp at symmetrical pH 7.4, ICALHM1 decreased at acidic pHe or pHi, whereas it sharply increased at alkaline pHe or pHi. The effects of pH were preserved in the inside-out configuration. The voltage dependence of ICALHM1 showed leftward and rightward shifts at alkaline and acidic pHe and pHi, respectively. Site-directed mutagenesis of the water-accessible charged residues of the pore and nearby domains revealed that E17, K229, E233, D257, and E259 are nonadditively responsible for facilitation at alkaline pHi. Identification of the pHe-sensing residue was not possible because mutation of putative residues impaired membrane expression, resulting in undetectable ICALHM1. Alkaline pHe-dependent facilitation appeared gradually with depolarization, suggesting that the sensitivity to pHe might be due to H+ diffusion through the open-state CALHM1. At pHe 6.2, decreased [Ca2+]e could not recover the inhibited ICALHM1 but further augmented the increased ICALHM1 at pHe 8.6, suggesting that unidentified common residues might contribute to the [Ca2+]e and acidic pHe. This study is the first, to our knowledge, to demonstrate the remarkable pH sensitivity of CALHM1, which might contribute to the pH-dependent modulation of neuronal excitability or taste sensation.