*These authors contributed equally.Acid-sensing ion channels (ASICs) and the epithelial Na + channel (ENaC) are both members of the ENaC/degenerin family of amiloride-sensitive Na + channels. ASICs act as proton sensors in the nervous system where they contribute, besides other roles, to fear behaviour, learning and pain sensation. ENaC mediates Na + reabsorption across epithelia of the distal kidney and colon and of the airways. ENaC is a clinically used drug target in the context of hypertension and cystic fibrosis, while ASIC is an interesting potential target. Following a brief introduction, here we will review selected aspects of ASIC and ENaC function. We discuss the origin and nature of pH changes in the brain and the involvement of ASICs in synaptic signalling. We expose how in the peripheral nervous system, ASICs cover together with other ion channels a wide pH range as proton sensors. We introduce the mechanisms of aldosterone-dependent ENaC regulation and the evidence for an aldosterone-independent control of ENaC activity, such as regulation by dietary K + . We then provide an overview of the regulation of ENaC by proteases, a topic of increasing interest over the past few years. In spite of the profound differences in the physiological and pathological roles of ASICs and ENaC, these channels share many basic functional and structural properties. It is likely that further research will identify physiological contexts in which ASICs and ENaC have similar or overlapping roles. AbbreviationsAQP, aquaporin; ASDN, aldosterone-sensitive distal nephron; ASIC, acid-sensing ion channel; BASIC, bile acid-activated ion channel; BK, big calcium-activated K + channel; CA, carbonic anhydrase; CAP-1, -2, or-3, channel activating proteases; Ca v , voltage-gated Ca2 + channel; CCD, cortical collecting duct; CD, collecting duct; PHA-1, pseudohypoaldosteronism type 1; CFTR, cystic fibrosis transmembrane conductance regulator; CNT, connecting tubule; CF, cystic fibrosis; CLCN/Kb, voltage-sensitive chloride channel Kb; DCT, distal convoluted tubule; DRG, dorsal root ganglion; ENaC, amiloride-sensitive epithelial sodium channel; EPSP, excitatory post-synaptic potential; FaNaC, FMRFa-activated Na + channel; FMRFa, PheMet-Arg-Phe-amide; GMQ, 2-guanidine-4-methylquinazoline; GPI, glycosylphosphatidyl-inositol; HAI, hepatocyte growth factor activator inhibitor; HCN, hyperpolarization-activated cyclic nucleotide-gated channel; I A , A-type current of rapid inactivating K + channels; iGluR, ionotropic glutamate receptor; I K , K + current; I Na , Na + current; I h , current produced by HCN channels; I max , maximal current amplitude; Kir, inward rectifier K + channel; K v , voltage-gated K + channel; MR, mineralocorticoid receptor; Na v , voltage-gated Na + channel; NBC, Na + , -HCO 3 -cotransporter; NCC, Na +-Cl À cotransporter; NHE, Na +-H + exchanger; OSR1/SPAK, Ste20-related protein kinases; P2X, purinergic receptor; PcTx1, Psalmotoxin 1; pH 50 , pH of half-maximal activation; pHe, extracellular pH; pH 50 Inh./Act., pH of h...

The epithelial sodium channel (ENaC) is responsible for Na(+) and fluid absorption across colon, kidney, and airway epithelia. Short palate lung and nasal epithelial clone 1 (SPLUNC1) is a secreted, innate defense protein and an autocrine inhibitor of ENaC that is highly expressed in airway epithelia. While SPLUNC1 has a bactericidal permeability-increasing protein (BPI)-type structure, its NH2-terminal region lacks structure. Here we found that an 18 amino acid peptide, S18, which corresponded to residues G22-A39 of the SPLUNC1 NH2 terminus inhibited ENaC activity to a similar degree as full-length SPLUNC1 (∼2.5 fold), while SPLUNC1 protein lacking this region was without effect. S18 did not inhibit the structurally related acid-sensing ion channels, indicating specificity for ENaC. However, S18 preferentially bound to the βENaC subunit in a glycosylation-dependent manner. ENaC hyperactivity is contributory to cystic fibrosis (CF) lung disease. Unlike control, CF human bronchial epithelial cultures (HBECs) where airway surface liquid (ASL) height was abnormally low (4.2 ± 0.6 μm), addition of S18 prevented ENaC-led ASL hyperabsorption and maintained CF ASL height at 7.9 ± 0.6 μm, even in the presence of neutrophil elastase, which is comparable to heights seen in normal HBECs. Our data also indicate that the ENaC inhibitory domain of SPLUNC1 may be cleaved away from the main molecule by neutrophil elastase, suggesting that it may still be active during inflammation or neutrophilia. Furthermore, the robust inhibition of ENaC by the S18 peptide suggests that this peptide may be suitable for treating CF lung disease.

Background:The small molecule GMQ opens ASIC3 channels by an unknown mechanism. Results: GMQ alters pH-dependent gating in all ASICs and amplifies an existing sustained current in ASIC3. Conclusion:The consequences of GMQ binding on channel function are ASIC subtype-dependent. Significance: This study describes the complex effects of a novel class of small molecules on ASIC function.

Epithelial Na(+) channel (ENaC)/degenerin family members are involved in mechanosensation, blood pressure control, pain sensation, and the expression of fear. Several of these channel types display a form of desensitization that allows the channel to limit Na(+) influx during prolonged stimulation. We used site-directed mutagenesis and chemical modification, functional analysis, and molecular dynamics simulations to investigate the role of the lower palm domain of the acid-sensing ion channel 1, a member of the ENaC/degenerin family. The lower palm domains of this trimeric channel are arranged around a central vestibule, at ∼20 Å above the plasma membrane and are covalently linked to the transmembrane channel parts. We show that the lower palm domains approach one another during desensitization. Residues in the palm co-determine the pH dependence of desensitization, its kinetics, and the stability of the desensitized state. Mutations of palm residues impair desensitization by preventing the closing movement of the palm. Overexpression of desensitization-impaired channel mutants in central neurons allowed--in contrast to overexpression of wild type--a sustained signaling response to rapid pH fluctuations. We identify and describe here the function of an important regulatory domain that most likely has a conserved role in ENaC/degenerin channels.

Acid-sensing ion channels (ASICs) are H +-activated neuronal Na + channels. They are involved in fear behavior, learning, neurodegeneration after ischemic stroke and in pain sensation. ASIC activation has so far been studied only with fast pH changes, although the pH changes associated with many roles of ASICs are slow. It is currently not known whether slow pH changes can open ASICs at all. Here, we investigated to which extent slow pH changes can activate ASIC1a channels and induce action potential signaling. To this end, ASIC1a current amplitudes and charge transport in transfected Chinese hamster ovary cells, and ASIC-mediated action potential signaling in cultured cortical neurons were measured in response to defined pH ramps of 1-40 s duration from pH 7.4 to pH 6.6 or 6.0. A kinetic model of the ASIC1a current was developed and integrated into the Hodgkin-Huxley action potential model. Interestingly, whereas the ASIC1a current amplitude decreased with slower pH ramps, action potential firing was higher upon intermediate than fast acidification in cortical neurons. Indeed, fast pH changes (<4 s) induced short action potential bursts, while pH changes of intermediate speed (4-10 s) induced longer bursts. Slower pH changes (>10 s) did in many experiments not generate action potentials. Computer simulations corroborated these observations. We provide here the first description of ASIC function in response to defined slow pH changes. Our study shows that ASIC1a currents, and neuronal activity induced by ASIC1a currents, strongly depend on the speed of pH changes. Importantly, with pH changes that take >10 s to complete, ASIC1a activation is inefficient. Therefore, it is likely that currently unknown modulatory mechanisms allow ASIC activity in situations such as ischemia and inflammation.