MABN2521-25UL Sigma-AldrichAnti-CALHM1 Antibody, clone 32C2

Conventional chemical synapses in the nervous system involve a presynaptic accumulation of neurotransmitter-containing vesicles, which fuse with the plasma membrane to release neurotransmitters that activate postsynaptic receptors. In taste buds, type II receptor cells do not have conventional synaptic features but nonetheless show regulated release of their afferent neurotransmitter, ATP, through a large-pore, voltage-gated channel, CALHM1. Immunohistochemistry revealed that CALHM1 was localized to points of contact between the receptor cells and sensory nerve fibers. Ultrastructural and super-resolution light microscopy showed that the CALHM1 channels were consistently associated with distinctive, large (1- to 2-μm) mitochondria spaced 20 to 40 nm from the presynaptic membrane. Pharmacological disruption of the mitochondrial respiratory chain limited the ability of taste cells to release ATP, suggesting that the immediate source of released ATP was the mitochondrion rather than a cytoplasmic pool of ATP. These large mitochondria may serve as both a reservoir of releasable ATP and the site of synthesis. The juxtaposition of the large mitochondria to areas of membrane displaying CALHM1 also defines a restricted compartment that limits the influx of Ca2+ upon opening of the nonselective CALHM1 channels. These findings reveal a distinctive organelle signature and functional organization for regulated, focal release of purinergic signals in the absence of synaptic vesicles.

Calcium homeostasis modulator 1 (CALHM1) is a Ca(2+) 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 Ca(2+) 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 Ca(2+)-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, Zn(2+), and Gd(3+), or expression of the CALHM1 N140A and W114A mutants, which are deficient in mediating Ca(2+) 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 Ca(2+)-dependent ERK1/2 signaling in neurons, and further establishes CALHM1 as a critical ion channel for neuronal signaling and function.