Acid-sensing ion channels (ASICs) are cationic channels activated by extracellular protons. They are expressed in sensory neurons, where they are thought to be involved in pain perception associated with tissue acidosis. They are also expressed in brain. A number of brain regions, like the hippocampus, contain large amounts of chelatable vesicular Zn 2؉ . This paper shows that Zn 2؉ potentiates the acid activation of homomeric and heteromeric ASIC2a-containing channels (i.e. ASIC2a, ASIC1a؉2a, ASIC2a؉3), but not of homomeric ASIC1a and ASIC3. The EC 50 for Zn 2؉ potentiation is 120 and 111 M for the ASIC2a and ASIC1a؉2a current, respectively. Zn 2؉ shifts the pH dependence of activation of the ASIC1a؉2a current from a pH 0.5 of 5.5 to 6.0. Systematic mutagenesis of the 10 extracellular histidines of ASIC2a leads to the identification of two residues (His-162 and His-339) that are essential for the Zn 2؉ potentiating effect. Mutation of another histidine residue, His-72, abolishes the pH sensitivity of ASIC2a. This residue, which is located just after the first transmembrane domain, seems to be an essential component of the extracellular pH sensor of ASIC2a.

Amiloride-sensitive Na + channels belonging to the recently discovered NaC/DEG family of genes have been found in several human tissues including epithelia and central and peripheral neurons. We describe here the molecular cloning of a cDNA encoding a novel human amiloride-sensitive Na + channel subunit that is principally expressed in the small intestine and has been called hINaC (human intestine Na + channel). This protein is similar to the recently identified rodent channel BLINaC and is relatively close to the acid sensing ion channels (ASICs) (79 and 29% amino acid identity, respectively). ASICs are activated by extracellular protons and probably participate in sensory neurons to nociception linked to tissue acidosis. hINaC is not activated by lowering the external pH but gain-of-function mutations can be introduced and reveal when expressed in Xenopus oocytes, an important Na + channel activity which is blocked by amiloride (IC 50 = 0.5 W WM). These results suggest the existence of a still unknown physiological activator for hINaC (e.g. an extracellular ligand). The presence of this new amiloride-sensitive Na + channel in human small intestine probably has interesting physiological as well as physiopathological implications that remain to be clarified. The large activation of this channel by point mutations may be associated with a degenerin-like behavior as previously observed for channels expressed in nematode mechanosensitive neurons. The hINaC gene has been mapped on the 4q31.3^q32 region of the human genome.z 2000 Federation of European Biochemical Societies.

Acid-sensing ion channels (ASICs) are cationic channels activated by extracellular pH. They are present in the brain, where they are thought to participate in signal transduction associated with local pH variations, and in sensory neurons, where they have been involved in pain perception associated with tissue acidosis and in mechanoperception. The ASIC3 subunit is mainly expressed in dorsal root ganglion neurons. Its expression is associated with a rapidly inactivating current followed by a slowly activating sustained current thought to be required for the tonic sensation of pain caused by acids. We report here the interaction of this channel subunit with the multivalent PDZ (PSD-95 Drosophila discs-large protein, Zonula occludens protein 1) domaincontaining protein CIPP. This interaction requires the C-terminal region of ASIC3 and the fourth PDZ domain of CIPP. Co-expression of CIPP and ASIC3 in COS cells increases the maximal ASIC3 peak current density by a factor of 5 and slightly shifts the pH 0.5 for activation from pH 6.2 to pH 6.4. CIPP mRNA is found at a significant level in the same dorsal root ganglion neuronal cell population that expresses the ASIC3 subunit, i.e. mainly in the small nociceptive neurons. CIPP is thus a scaffolding protein that could both enhance the surface expression of ASIC3 and bring together ASIC3 and functionally related proteins in the membrane of sensory neurons.Proton-gated Na ϩ -permeable channels have been described in many neuronal cell types in the central nervous system as well as in nociceptive neurons (1). In the sensory neurons, they are thought to be responsible for the sensation of pain that accompanies tissue acidosis for instance in muscle and cardiac ischemia (2, 3) and in inflammation (4, 5). Their function in the central nervous system is less documented, but an important role for these channels in signal transduction associated with local pH variations during normal neuronal activity (6, 7) has been proposed (8). They might also be involved in pathological situations such as brain ischemia and epilepsy that produce significant extracellular acidification. A clear correlation between native H ϩ -gated currents and the recently cloned acidsensing ion channels (ASICs) 1 (9) has been established in both sensory neurons (5, 10 -12) and central neurons (13,14).ASICs are proton-gated Na ϩ -permeable channels comprising four different genes encoding six polypeptides: ASIC1a (15) and ASIC1b (16, 17), ASIC2a (18 -21) and ASIC2b (22), ASIC3 (23-25), and the putative ASIC subunit ASIC4 (26, 27) that is not activated by acidic pH when expressed alone. They can form functional homomeric and heteromeric ligand-gated cationic channels with different kinetics, external pH sensitivities, and tissue distribution (22,28,29). ASIC1a and ASIC2b are present in brain and afferent sensory neurons, whereas the splice or initiation variant ASIC1b is found only in sensory neurons. ASIC1a and ASIC1b both mediate fast inactivating currents upon modest but rapid acidification of the externa...