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Summary Calcium homeostasis modulator 1 (CALHM1) is a Ca2+ channel controlling neuronal excitability and potentially involved in the pathogenesis of Alzheimer's disease (AD). Although strong evidence indicates that CALHM1 is required for neuronal electrical activity, its role in intracellular Ca2+ signaling remains unknown. In the present study, we show that in hippocampal HT-22 cells, CALHM1 expression led to a robust and relatively selective activation of the Ca2+-sensing kinases ERK1/2. CALHM1 also triggered activation of MEK1/2, the upstream ERK1/2-activating kinases, and of RSK1/2/3 and MSK1, two downstream effectors of ERK1/2 signaling. CALHM1-mediated activation of ERK1/2 signaling was controlled by the small GTPase Ras. Pharmacological inhibition of CALHM1 permeability using Ruthenium Red, Zn2+, and Gd3+, or expression of the CALHM1 N140A and W114A mutants, which are deficient in mediating Ca2+ influx, prevented the effect of CALHM1 on the MEK, ERK, RSK and MSK signaling cascade, demonstrating that CALHM1 controlled this pathway via its channel properties. Importantly, expression of CALHM1 bearing the natural P86L polymorphism, which leads to a partial loss of CALHM1 function and is associated with an earlier age at onset in AD patients, showed reduced activation of ERK1/2, RSK1/2/3, and MSK1. In line with these results obtained in transfected cells, primary cerebral neurons isolated from Calhm1 knockout mice showed significant impairments in the activation of MEK, ERK, RSK and MSK signaling. The present study identifies a previously uncharacterized mechanism of control of Ca2+-dependent ERK1/2 signaling in neurons, and further establishes CALHM1 as a critical ion channel for neuronal signaling and function.

Afferent output in type II taste cells is mediated by ATP liberated through ion channels. It is widely accepted that pannexin 1 (Panx1) channels are responsible for ATP release in diverse cell types, including taste cells. While biophysical evidence implicates slow deactivation of ion channels following ATP release in taste cells, recombinant Panx1 activates and deactivates rapidly. This inconsistency could indicate that the cellular context specifies Panx1 functioning. We cloned Panx1 from murine taste tissue, and heterologously expressed it in three different cell lines: HEK-293, CHO and neuroblastoma SK-N-SH cells. In all three cell lines, Panx1 transfection yielded outwardly rectifying anion channels that exhibited fast gating and negligible permeability to anions exceeding 250 Da. Despite expression of Panx1, the host cells did not liberate ATP upon stimulation, making it unclear whether Panx1 is involved in taste-related ATP secretion. This issue was addressed using mice with genetic ablation of the Panx1 gene. The ATP-biosensor assay revealed that, in taste cells devoid of Panx1, ATP secretion was robust and apparently unchanged compared with the control. Our data suggest that Panx1 alone forms a channel that has insufficient permeability to ATP. Perhaps, a distinct subunit and/or a regulatory circuit that is absent in taste cells is required to enable a high ATP-permeability mode of a native Panx1-based channel.