Biophysical Properties of Calcium Homeostasis Modulator 1 (CALHM1) Ion Channel

Abstract

CALHM1 was identified recently as a membrane protein expressed in the hippocampus and linked to late onset Alzheimer's disease (Cell,133:1149(2008)). Here, we demonstrate its permeation and gating properties in plasma membrane. The relative permeability sequence determined from reversal potentials is: PNa+: PK+ : PCa2+ : PCl− = 1: 1.2: 4.4: 0.52, indicating that CALHM1 is a Ca2+-permeable channel. Inward currents elicited by lowering extracellular Ca2+ concentration ([Ca2+]o) were inhibited by Gd3+, but not by blockers of connexin or NMDA receptors. CALHM1 channels are activated by voltage and by lowering [Ca2+]o with an IC50 of 250 μM at a holding potential of −15 mV. 5 mM Ca2+ shifts the G-V relation by +150 mV and increases the slope (Ze) by 3-fold without reducing Gmax (Boltzman fits in 0 Ca2+: V0.5 = −70 mV, Ze= 0.54; in 5 mM Ca2+: V0.5 =+82 mV, Ze=1.48), indicating that CALHM1 has an intrinsic voltage-dependent gate, although it lacks an S4-like domain, and that Ca2+o regulates voltage-dependent gating by voltage-dependent conformation changes rather than by voltage-dependent blockage, with high selectivity for Ca2+ over Mg2+ (IC50 0.25 vs 3 mM). Mild oxidation of CALHM1 by 0.1% H2O2 alters this Ca2+-regulation, resulting in channel more leaky at physiological conditions. Biochemical and single molecule bleaching measurements suggest that CALHM1 is oligomeric with six monomers comprising the pore. Together these properties suggest that CALHM1 is a novel Ca2+-permeable cation channel, which is regulated by voltage, Ca2+o and oxidative stress. Insights into the properties of CALHM1 channels may help us to understand the mechanism of Ca2+ influx through CALHM1 in physiological and pathological conditions, and to facilitate therapeutic interventions in Alzheimer's disease.