A single amino acid substitution in MDEG2 specifically alters desensitization of the proton-activated cation current

An arginine residue in the outer segment of hASIC1a TM1 affects both proton affinity and channel desensitization

Acid-sensing ion channels (ASICs) respond to changes in pH in the central and peripheral nervous systems and participate in synaptic plasticity and pain perception. Understanding the proton-mediated gating mechanism remains elusive despite the of their structures in various conformational states. We report here that R64, an arginine located in the outer segment of the first transmembrane domain of all three isoforms of mammalian ASICs, markedly impacts the apparent proton affinity of activation and the degree of desensitization from the open and preopen states. Rosetta calculations of free energy changes predict that substitutions of R64 in hASIC1a by aromatic residues destabilize the closed conformation while stabilizing the open conformation. Accordingly, F64 enhances the efficacy of proton-mediated gating of hASIC1a, which increases the apparent pH₅₀ and facilitates channel opening when only one or two subunits are activated. F64 also lengthens the duration of opening events, thus keeping channels open for extended periods of time and diminishing low pH-induced desensitization. Our results indicate that activation of a proton sensor(s) with pH₅₀ equal to or greater than pH 7.2–7.1 opens F64hASIC1a, whereas it induces steady-state desensitization in wildtype channels due to the high energy of activation imposed by R64, which prevents opening of the pore. Together, these findings suggest that activation of a high-affinity proton-sensor(s) and a common gating mechanism may mediate the processes of activation and steady-state desensitization of hASIC1a.

Acid-sensing ion channels in taste buds

Taste receptor cells detect gustatory stimuli using a complex arrangement of ion channels, G protein-coupled receptors, and signaling cascades. Sour and salty tastes are detected by ion channels in the rat. Using a combination of homology screening and functional expression approaches, we screened a rat circumvallate papilla cDNA library and identified acid-sensing ion channel-2a (ASIC2a) and ASIC2b as candidates for the rat sour-sensing channels. In situ hybridization and reverse transcription-polymerase chain reaction experiments revealed that ASIC2a and ASIC2b transcripts were localized in taste bud cells. Immunohistochemistry and immunoprecipitation also revealed that both subunits were expressed in a subset of taste cells and that some of the cells expressed ASIC2a/ASIC2b heteromeric assemblies. Electrophysiological studies demonstrated that stimulation of acetic acid produced larger ASIC2 currents than did hydrochloric acid at the same pH. ASIC2a/ ASIC2b channels generated maximal inward currents at pH <or= 2.0, which agrees well with the in vivo pH-sensitivity of rat taste cells. The amiloride-sensitivity of ASIC2a/ ASIC2b heteromer lessened with decreasing pH and almost completely disappeared at pH 2.0. These data suggest that ASIC2a and ASIC2b may play roles in sour taste transduction.

Protons activate the delta-subunit of the epithelial Na+ channel in humans

The amiloride-sensitive epithelial Na(+) channel (ENaC) controls Na(+) transport into cells and across epithelia. So far, four homologous subunits of mammalian ENaC have been isolated and are denoted as alpha, beta, gamma, and delta. ENaCdelta can associate with beta and gamma subunits and generate a constitutive current that is 2 orders of magnitude larger than that of homomeric ENaCdelta. However, the distribution pattern of ENaCdelta is not consistent with that of the beta and gamma subunits. ENaCdelta is expressed mainly in the brain in contrast to beta and gamma subunits, which are expressed in non-neuronal tissues. To explain this discrepancy, we searched for novel functional properties of homomeric ENaCdelta and investigated the detailed tissue distribution in humans. When human ENaCdelta was expressed in Xenopus oocytes and Chinese hamster ovary cells, a reduction of extracellular pH activated this channel (half-maximal pH for an activation of 5.0), and the acid-induced current was abolished by amiloride. The most striking finding was that the desensitization of the acid-evoked current was much slower (by approximately 10% 120 s later), dissociating from the kinetics of acid-sensing ion channels in the degenerin/epithelial Na(+) channel family, which were rapidly desensitized during acidification. RNA dot-blot analyses showed that ENaCdelta mRNA was widely distributed throughout the brain and was also expressed in the heart, kidney, and pancreas in humans. Northern blotting confirmed that ENaCdelta was expressed in the cerebellum and the hippocampus. In conclusion, human ENaCdelta activity is regulated by protons, indicating that it may contribute to the pH sensation and/or pH regulation in the human brain.

Identification of sour-taste receptor genes

Taste cells located in taste buds respond to gustatory stimuli using a complex arrangement of ion channels, receptor molecules and signaling cascades. Previous electrophysiological experiments have shown that sour taste (essentially a taste of protons) is mediated, at least in part, by apically located amiloride-sensitive channels in the rat. Here, the molecular cloning of sour-taste receptor genes is described. Using a combination of homology screening and functional expression approaches, we screened a rat circumvallate papilla cDNA library and identified acid-sensing ion channel-2a (ASIC2a) and ASIC2b, amiloride-sensitive proton-activated cation channels. In situ hybridization and reverse transcription-polymerase chain reaction experiments showed that ASIC2a and ASIC2b transcripts were localized in taste cells. Immunohistochemical and immunoprecipitation studies revealed that both channels were expressed in a subset of taste cells and that some of the cells expressed ASIC2a/ASIC2b heteromeric assemblies. Immunoelectron microscopic analyses demonstrated that some of the ASIC2a-immunopositive cells had the characteristics of type III cells, which make synaptic contacts with gustatory afferent neurons. According to our electrophysiological studies, stimulation by acetic acid generated larger inward currents in ASIC2a- or ASIC2a/ASIC2b-expressing oocytes than those induced by hydrochloric acid at the same proton concentration and these findings are in good agreement with the well-known taste phenomenon that acetic acid is more sour than hydrochloric acid at equal pH. Taken together, the present results strongly suggest that mucosal protons dissociated from sour-taste substances induce taste cell depolarization through the activation of the ASIC2a and ASIC2a/ASIC2b channels, which leads to transmitter release onto gustatory neurons.

Amiloride-insensitive currents of the acid-sensing ion channel-2a (ASIC2a)/ASIC2b heteromeric sour-taste receptor channel

Acid-sensing ion channel-2a (ASIC2a) is an amiloride-blockable proton-gated cation channel, probably contributing to sour-taste detection in rat taste cells. To isolate another subtype of the sour-taste receptor, we screened a rat circumvallate papilla cDNA library and identified ASIC2b, an N-terminal splice variant of ASIC2a. Reverse transcription-PCR analyses confirmed the expression of ASIC2b transcripts in the circumvallate papilla and, moreover, demonstrated its expression in the foliate and fungiform papillae. Immunohistochemical analyses revealed that ASIC2b, as well as ASIC2a, was expressed in a subpopulation of taste cells in the circumvallate, foliate, and fungiform papillae, and some of the cells displayed both ASIC2a and ASIC2b immunoreactivities. Subsequent coimmunoprecipitation studies with circumvallate papillae extracts indicated that ASIC2b associated with ASIC2a to form assemblies and, together with our immunohistochemical findings, strongly suggested that both ASIC2 subunits formed heteromeric channels in taste cells in the circumvallate, foliate, and fungiform papillae. Oocyte electrophysiology demonstrated that the ASIC2a/ASIC2b channel generated maximal inward currents at a pH of < or =2.0, which is in agreement with the in vivo pH sensitivity of rat taste cells, and that the amiloride sensitivity of the heteromer decreased with decreasing pH and was almost completely abolished at a pH of 2.0. These findings provide persuasive explanations for the amiloride insensitivity of acid-induced responses of rat taste cells.

Amiloride-blockable acid-sensing ion channels are leading acid sensors expressed in human nociceptors

Many painful inflammatory and ischemic conditions such as rheumatoid arthritis, cardiac ischemia, and exhausted skeletal muscles are accompanied by local tissue acidosis. In such acidotic states, extracellular protons provoke the pain by opening cation channels in nociceptors. It is generally believed that a vanilloid receptor subtype-1 (VR1) and an acid-sensing ion channel (ASIC) mediate the greater part of acid-induced nociception in mammals. Here we provide evidence for the involvement of both channels in acid-evoked pain in humans and show their relative contributions to the nociception. In our psychophysical experiments, direct infusion of acidic solutions (pH > or = 6.0) into human skin caused localized pain, which was blocked by amiloride, an inhibitor of ASICs, but not by capsazepine, an inhibitor of VR1. Under more severe acidification (pH 5.0) amiloride was less effective in reducing acid-evoked pain. In addition, capsazepine had a partial blocking effect under these conditions. Amiloride itself neither blocked capsaicin-evoked localized pain in human skin nor inhibited proton-induced currents in VR1-expressing Xenopus oocytes. Our results suggest that ASICs are leading acid sensors in human nociceptors and that VR1 participates in the nociception mainly under extremely acidic conditions.

Epithelial sodium channel/degenerin family of ion channels: a variety of functions for a shared structure

The recently discovered epithelial sodium channel (ENaC)/degenerin (DEG) gene family encodes sodium channels involved in various cell functions in metazoans. Subfamilies found in invertebrates or mammals are functionally distinct. The degenerins in Caenorhabditis elegans participate in mechanotransduction in neuronal cells, FaNaC in snails is a ligand-gated channel activated by neuropeptides, and the Drosophila subfamily is expressed in gonads and neurons. In mammals, ENaC mediates Na+ transport in epithelia and is essential for sodium homeostasis. The ASIC genes encode proton-gated cation channels in both the central and peripheral nervous system that could be involved in pain transduction. This review summarizes the physiological roles of the different channels belonging to this family, their biophysical and pharmacological characteristics, and the emerging knowledge of their molecular structure. Although functionally different, the ENaC/DEG family members share functional domains that are involved in the control of channel activity and in the formation of the pore. The functional heterogeneity among the members of the ENaC/DEG channel family provides a unique opportunity to address the molecular basis of basic channel functions such as activation by ligands, mechanotransduction, ionic selectivity, or block by pharmacological ligands.