1.A.6 The Epithelial Na+ Channel (ENaC) Family

Epithelial sodium channels facilitate Na⁺ reabsorption across the apical membranes of epithelia in the distal nephron, respiratory and reproductive tracts and exocrine glands, and hence they have a role in fluid volume homeostasis, osmolarity and arterial blood pressure regulation (Enuka et al. 2012). Acid-sensing ion channels are broadly distributed in the nervous system where they contribute to sensory processes (Schuhmacher et al. 2015). ENaC family members are from animals with no recognizable homologues in other eukaryotes or bacteria. The vertebrate ENaC proteins from epithelial cells cluster tightly together on the phylogenetic tree; voltage-insensitive ENaC homologues are also found in the brain. The many sequenced C. elegans proteins, including the worm degenerins, are distantly related to the vertebrate proteins as well as to each other. At least some of these proteins form part of a mechano-transducing complex for touch sensitivity, but others function in chemosensory transduction pathways (Ben-Shahar, 2011). D. melanogaster also has many ENaC family paralogues, some closely related to each other, others very distant in sequence. Other members of the ENaC family, the acid-sensing and/or mechanosensory ion channels, ASIC1-4, are homo- or hetero-oligomeric neuronal Zn2+ and H+-gated, mechanosensitive channels that mediate pain sensation in response to tissue acidosis. Two extracellular histidines (his-162 and his-339) potentiate Zn2+ activation while another (his-72) mediates pH sensitivity (Baron et al., 2001). ASIC1-4 also mediate light touch sensation and are excited by hair movement. The homologous Helix aspersa (FMRF-amide)-activated Na+ channel is the first peptide neurotransmitter-gated ionotropic receptor to be sequenced. Salty taste is mediated by an ENaC channel in the fungiform papillae in the dorsal epithelium of the anterior tongue. Activation of acid-sensing ion channel 1a (ASIC1a) occurs in response to surface trafficking (Chai et al., 2010). The stress response protein, SERP1, regulates ENaC biogenesis (Faria et al., 2012). Potassium activates mTORC2-dependent SGK1 phosphorylation to stimulate ENaC (Saha et al. 2023). The epithelial sodium channel is a drug target (Lemmens-Gruber and Tzotzos 2023). The optimization of electrochemical immunosensors can be used to detect epithelial sodium channel as a biomarker of hypertension (Lestari et al. 2023). 6-Iodoamiloride is an amiloride analog, a potent inhibitor of ASICs (Finol-Urdaneta et al. 2023).

Epithelial Na+ channel (ENaC)/degenerin family members are involved in mechanosensation, blood pressure control, pain sensation, and the expression of fear.  They display a form of desensitization (Roy et al. 2013).  Members all exhibit the same apparent topology, each with N- and C-termini on the inside of the cell, two amphipathic transmembrane spanning segments, M1 and M2, and a large extracellular loop (Saugstad et al. 2004). The extracellular domains contain numerous highly conserved cysteine residues. They are proposed to serve a receptor function. Welsh et al. (2002) presented three models whereby members of the ENaC family sense mechanostimulation. Their preferred model involves tethering the channel protein to extracellular matrix proteins such as collagens and/or intracellular cystoskeletal proteins such as α- and β-tubulins. Carnally et al., 2008 have presented evidence, based on an X-ray crystal structure, that ASIC1a assembles as a heterotrimer. Carattino (2011) has reviewed the structural mechanisms underlying the functions of epithelial sodium channel/acid-sensing ion channels. Opening of the ion conductive pathway involves coordinated rotation of the second transmembrane-spanning domains (Tolino et al., 2011). The second TMS modulates channel gating in response to shear stress (Abi-Antoun et al., 2011).  ASIC- and ENaC-types of Na+ channels exhibit different conformational changes (Hanukoglu 2016).  The ion selectivity filter has been discussed (Hanukoglu 2016).  Interactions between the epithelial sodium channel gamma-subunit and claudin-8 modulates paracellular sodium permeability in the renal collecting duct (Sassi et al. 2020). Reactive species generated by heme impair alveolar epithelial sodium channel function in acute respiratory distress syndrome (Aggarwal et al. 2020). DEG/ENaC/ASIC channels vary in their sensitivities to anti-hypertensive and non-steroidal anti-inflammatory drugs (Fechner et al. 2021).  Epithelial sodium channels exhibit sodium self-inhibition (Lawong et al. 2023).

Mammalian ENaC is important for the maintenance of Na+ balance and the regulation of blood pressure. Three homologous ENaC subunits, α, β and γ, have been shown to assemble to form the highly Na+-selective channel. Only the dehydrated form of Na+ (or Li+) is transported. The stoichiometry of the three subunits is αβγ in a heterotrimeric architecture, and they form a triangular pyramid-shaped funnel (Edelheit et al. 2014). A structural model has been proposed in which the properties of the channel are conferred by the second TMS together with the preceding hydrophobic region that may loop into the membrane as do the P-regions of VIC family members. The selectivity filter of the epithelial Na+ channel α-subunit is at least in part determined by residues Ser580 to Ser592 following the second TMS. Residues conferring cation selectivity are in both M2 and the preceding loop. Negatively charged residues in M2 of the mammalian α-subunit are important, as two substitutions, αE595C and αD602C confer K+ permeability (Sheng et al., 2001b).

The C-terminus of each ENaC subunit contains a PPXY motif which when mutated or deleted in either the β- or γ-ENaC subunit leads to Liddle's syndrome, a human autosomal dominant form of hypertension. In this disease, the mutation induces abnormally high levels of channel expression due to a loss of interaction with the inhibitory Nedd4 protein. Nedd4 regulates the activity of the epithelial Na+ channel in normal people but not in those suffering from Liddle's syndrome. Multiple WW domains in Nedd4 mediate the interaction with all three subunits of ENaC, α, β and γ, and WW domains 2-4 are most important for this interaction (Snyder et al., 2001). Cys palmitoylation of the β subunit modulates gating of the epithelial sodium channel (Mueller et al., 2010).  Bile acids, especially tauro-deoxycholic acid (t-DCA), modify the function of the acid-sensing ion channel ASIC1a and other members of the epithelial sodium channel (ENaC)/degenerin (DEG) ion channel family (Ilyaskin et al. 2019).

Cystic fibrosis (CF) lung disease is caused by the loss of function of the cystic fibrosis transmembrane conductance regulator (CFTR) combined with hyperactivation of the epithelial sodium channel (ENaC). In the lung, ENaC is responsible for movement of sodium. Hyperactivation of ENaC, which creates an osmotic gradient that pulls fluid out of the airway, contributes to reduced airway hydration, causing mucus dehydration, decreased mucociliary clearance, and recurrent acute bacterial infections. ENaC represents a therapeutic target to treat patients with CF independently of their underlying CFTR mutation. SPX-101, a peptide resulting from proteolytic digestion of SPLUNC1 (Q9NP55; 256 aas) binds selectively to ENaC and promotes internalization of the α-, β-, and γ-subunits. Removing ENaC from the membrane with SPX-101 causes a significant decrease in amiloride-sensitive current, promting survial of CF patients (Scott et al. 2017).

Acid-sensing ion channels (ASICs) have been implicated in perception of pain, ischaemic stroke, mechanosensation, learning and memory. They are implicated in touch, pain, digestive function, baroreception, blood volume control and hearing (Chen and Wong 2013).  Jasti et al. (2007) reported the low-pH crystal structure of a chicken ASIC1 deletion mutant at 1.9 Å resolution. Each subunit of the chalice-shaped homotrimer is composed of short amino and carboxy termini, and two transmembrane helices. A bound chloride ion is present. A disulphide-rich, multidomain extracellular region is enriched in acidic residues with carboxyl-carboxylate pairs, suggesting that at least one carboxyl group bears a proton. Electrophysiological studies on aspartate-to-asparagine mutants confirmed that these carboxyl-carboxylate pairs participate in proton sensing. Between the acidic residues and the transmembrane pore lies a disulphide-rich 'thumb' domain poised to couple the binding of protons to the opening of the ion channel. The results demonstrated that proton activation involves long-range conformational changes. The Akt and Sgk protein kinases are components of an insulin signaling pathway that increases Na+ absorption by up-regulating membrane expression of ENaC via a regulatory system that involves inhibition of Nedd4-2 (Lee et al., 2007).

Gonzales et al. (2009) presented the structure of a functional acid-sensing ion channel in a desensitized state at 3 Å resolution, the location and composition of the approximately 8 Å thick desensitization gate, and the trigonal antiprism coordination of caesium ions bound in the extracellular vestibule. Comparison of the acid-sensing ion channel structure with the ATP-gated P2X(4) receptor revealed similarity in pore architecture and aqueous vestibules, suggesting that there are unanticipated yet common structural and mechanistic principles (Gonzales et al., 2009).  ENaCs have been used to form solid-state nanopores with diameters in the range of 150-200 nm and a thickness <1 micron which could serve as a platform to enhance the throughput of ion-channel characterization using Black Lipid Membranes (Khan et al. 2016). Acid-sensing ion channels (ASICs) are weakly sodium selective (sodium:potassium ratio approximately 10:1), while ENaCs show a high preference for sodium over potassium (>500:1). The pre-TMS1 and TMS1 regions of mASIC1a channels are major determinants of ion selectivity (Sheikh et al. 2021).

The activity of the epithelial sodium channel (ENaC) is modulated by multiple external factors, including proteases, cations, anions and shear stress. The resolved crystal structure of acid-sensing ion channel 1 (ASIC1), and mutagenesis studies suggest that the large extracellular region is involved in recognizing external signals that regulate channel gating. The thumb domain in the extracellular region of ASIC1 has a cylinder-like structure with a loop at its base that is in proximity to the tract connecting the extracellular region to the transmembrane domains. This loop has been proposed to have a role in transmitting proton-induced conformational changes within the extracellular region to the gate. Shi et al. (2011) examined whether loops at the base of the thumb domains within ENaC subunits have a similar role in transmitting conformational changes induced by external Na+ and shear stress. Mutations at selected sites within this loop in each of the subunits altered channel responses to both external Na+ and shear stress. The most robust changes were observed at the site adjacent to a conserved Tyr residue. In the context of channels that have a low open probability due to retention of an inhibitory tract, mutations in the loop activated channels in a subunit-specific manner. This loop may have a role in modulating channel gating in response to external stimuli, consistent with the hypothesis that external signals trigger movements within the extracellular regions of ENaC subunits that are transmitted to the channel gate (Shi et al., 2011).

As noted above, epithelial sodium channels (ENaC) consist of three homologous subunits. Channels composed solely of alpha and beta subunits (αβ-channels) exhibit a very high open probability (Po) and reduced sensitivity to amiloride, in contrast to channels composed of alpha and gamma subunits or of all three subunits (i.e., αγ- and αβγ-channels). A mutant channel comprised of alpha and beta subunits, and a chimeric gamma subunit where the region immediately preceding (beta12 and wrist) and encompassing the second transmembrane domain (TMS2) has been replaced with the corresponding region of the beta subunit (gamma-betaTMS2) and showed characteristics reminiscent of αβ-channels, including a reduced potency of amiloride block and a loss of Na+ self-inhibition (reflecting an increased Po) (Shi and Kleyman 2013). Substitutions at key pore-lining residues of the γβ-TMS2 chimera enhanced the Na+ self-inhibition response, whereas key γ-subunit substitutions reduced the response. Furthermore, multiple sites within the TMS2 domain of the γ-subunit were required to confer high amiloride potency. Thus, pore-lining residues in the γ-subunit are important for proper channel gating and its interaction with amiloride. 

Acid-sensing ion channels (ASICs) are cation selective proton-gated channels expressed in neurons that participate in diverse physiological processes including nociception, synaptic plasticity, learning, and memory. ASIC subunits contain intracellular N- and C- termini, two transmembrane domains that constitute the pore and a large extracellular loop with defined domains termed the finger, beta-ball, thumb, palm, and knuckle. Krauson and Carattino 2016 examined the contribution of the finger, beta-ball and thumb domains to activation and desensitization. The beta-ball and thumb domains reside apart in the resting state, but they become closer to each other in response to extracellular acidification. The thumb domain probably moves upon continuous exposure to an acidic extracellular milieu assisting with the closing of the pore during channel desensitization. 

The ENaC Family has been reviewed by Hanukoglu and Hanukoglu 2016.  ENaC dependent reabsorption of Na in kidney tubules regulates extracellular fluid (ECF) volume and blood pressure by modulating osmolarity. In multi-ciliated cells, ENaC is located in cilia and plays an essential role in the regulation of epithelial surface liquid volume necessary for cilial transport of mucus and gametes in the respiratory and reproductive tracts, respectively. The subunits that form ENaC (named as alpha, beta, gamma and delta, encoded by genes SCNN1A, SCNN1B, SCNN1G, and SCNN1D) are in the ENaC/Degenerin superfamily. The earliest appearance of ENaC orthologs is in the genomes of the most ancient vertebrate taxon, Cyclostomata (jawless vertebrates) including lampreys, followed by earliest representatives of Gnathostomata (jawed vertebrates) including cartilaginous sharks. Among Euteleostomi (bony vertebrates), Actinopterygii (ray finned-fishes) branch has lost ENaC genes. Yet, most animals in the Sarcopterygii (lobe-finned fish) branch including Tetrapoda, amphibians and amniotes (lizards, crocodiles, birds, and mammals), have four ENaC paralogs (Hanukoglu and Hanukoglu 2016).

ENaC subunits are subject to numerous posttranslational modifications, including glycosylation, protease activation, disulfide bond formation and palmitoylation, each of which modulates channel function. For example, glycan addition is regulated by sodium and affects protease activation at the cell surface, protein trafficking, sodium-dependent regulation, and sodium transport. Glycosylation of the alpha subunit also determines whether a chaperone, Lhs1/GRP170, selects the protein for endoplasmic reticulum-associated degradation. Recognition by this chaperone is blocked by assembly of the ENaC transmembrane domains. In contrast, cytosolic lysines are acetylated in the early secretory pathway, which inhibits ubiquitination and endocytosis at the cell surface (Buck and Brodsky 2018). 

ENaC exhibits a very high selectivity for Na+ over other cations, including K+, and this selectivity greatly exceeds that of the closely related acid-sensing channels (ASICs). Yang and Palmer 2018 assessed the roles of two regions of the ENaC transmembrane pore in the determination of cation selectivity. Mutations of conserved amino acids with acidic side chains near the cytoplasmic end of the pore diminish macroscopic currents but do not decrease the selectivity of the channel for Na+ versus K+. In the WT channel, voltage-dependent block of Na+ currents by K+ or guanidinium+, neither of which have detectable conductance, suggested that these ions permeate only approximately 20% of the transmembrane electric field. The site of K+ block appears to be nearer the extracellular end of the pore, close to a putative selectivity filter, but.while this region affects the Li:Na selectivity, the high Na:K selectivity was maintained. Yang and Palmer 2018 concluded that a different part of the pore constitutes the selectivity filter in ENaC versus ASIC.

ENaC regulates Na+ and water homeostasis. These heterotrimeric channels harbor protease-sensitive domains critical for gating. Noreng et al. 2018 presented the structure of human ENaC in the uncleaved state as determined by single-particle cryo-EM. The ion channel is composed of a large extracellular domain and a narrow transmembrane domain. ENaC assembles with a 1:1:1 stoichiometry of alpha:beta:gamma subunits arranged in a counter-clockwise manner. The shape of each subunit is reminiscent of a hand with key gating domains of a 'finger' and a 'thumb'. Wedged between these domains is the protease-sensitive inhibitory domain poised to regulate conformational changes of the 'finger' and 'thumb'.

Despite the sequence homology between acid-sensing ion channels (ASICs) and epithelial sodium channel (ENaCs), these channels display very different functional characteristics. Whereas ASICs are gated by protons and show a relatively low degree of selectivity for sodium over potassium, ENaCs are constitutively active and display a remarkably high degree of sodium selectivity.  Differences in the transmembrane domains of these ion channels help explain some of their distinct functional properties (Kasimova et al. 2019).

The generalized transport reaction for Na+ channels is:

Na+ (out)   Na+ (in).

That for the degenerins is:

Cation (out)   cation (in).



This family belongs to the ENaC/P2X Superfamily.

 

References:

Abi-Antoun, T., S. Shi, L.A. Tolino, T.R. Kleyman, and M.D. Carattino. (2011). Second transmembrane domain modulates epithelial sodium channel gating in response to shear stress. Am. J. Physiol. Renal Physiol 300: F1089-1095.

Adams, C.M., M.G. Anderson, D.G. Motto, M.P. Price, W.A. Johnson, and M.J. Welsh. (1998). Ripped pocket and pickpocket, novel Drosophila DEG/ENaC subunits expressed in early development and in mechanosensory neurons. J. Cell Biol. 140: 143-152.

Aggarwal, S., A. Lazrak, I. Ahmad, Z. Yu, A. Bryant, J.A. Mobley, D.A. Ford, and S. Matalon. (2020). Reactive species generated by heme impair alveolar epithelial sodium channel function in acute respiratory distress syndrome. Redox Biol 36: 101592. [Epub: Ahead of Print]

Alvarez de la Rosa, D., C.M. Canessa, G.K. Fyfe, and P. Zhang. (2000). Structure and regulation of amiloride-sensitive sodium channels. Annu. Rev. Physiol. 62: 573-594.

Ananchenko, A. and M. Musgaard. (2023). Multiscale molecular dynamics simulations predict arachidonic acid binding sites in human ASIC1a and ASIC3 transmembrane domains. J Gen Physiol 155:.

Arteaga, M.F., T. Coric, C. Straub, and C.M. Canessa. (2008). A brain-specific SGK1 splice isoform regulates expression of ASIC1 in neurons. Proc. Natl. Acad. Sci. U.S.A. 105: 4459-4464.

Baron, A., L. Schaefer, E. Lingueglia, G. Champigny, and M. Lazdunski. (2001). Zn2+ and H+ are coactivators of acid-sensing ion channels. J. Biol. Chem. 276: 35361-35367.

Ben-Shahar, Y. (2011). Sensory functions for degenerin/epithelial sodium channels (DEG/ENaC). Adv Genet 76: 1-26.

Bianchi L. (2007). Mechanotransduction: touch and feel at the molecular level as modeled in Caenorhabditis elegans. Mol Neurobiol. 36: 254-271.

Buck, T.M. and J.L. Brodsky. (2018). Epithelial sodium channel biogenesis and quality control in the early secretory pathway. Curr Opin Nephrol Hypertens 27: 364-372.

Canessa, C.M., L. Schild, G. Buell, B. Thorens, I. Gautschi, J.-D. Horisberger, and B.C. Rossier. (1994). Amiloride-sensitive epithelial Na+ channel is made of three homologous subunits. Nature 367: 463-467.

Carattino, M.D. (2011). Structural mechanisms underlying the function of epithelial sodium channel/acid-sensing ion channel. Curr Opin Nephrol Hypertens 20: 555-560.

Carattino, M.D. and M.C. Della Vecchia. (2012). Contribution of residues in second transmembrane domain of ASIC1a protein to ion selectivity. J. Biol. Chem. 287: 12927-12934.

Carnally, S.M., H.S. Dev, A.P. Stewart, N.P. Barrera, M.X. Van Bemmelen, L. Schild, R.M. Henderson, and J.M. Edwardson. (2008). Direct visualization of the trimeric structure of the ASIC1a channel, using AFM imaging. Biochem. Biophys. Res. Commun. 372: 752-755.

Chai, S., M. Li, D. Branigan, Z.G. Xiong, and R.P. Simon. (2010). Activation of acid-sensing ion channel 1a (ASIC1a) by surface trafficking. J. Biol. Chem. 285: 13002-13011.

Chelur, D.S., Ernstrom, G.G., M.B. Goodman, C.A. Yao, L. Chen, R. O'Hagan, and M. Chalfie. (2002). The mechanosensory protein MEC-6 is a subunit of the C. elegans touch-cell degenerin channel. Nature 420: 669-673.

Chen, C.C. and C.W. Wong. (2013). Neurosensory mechanotransduction through acid-sensing ion channels. J Cell Mol Med 17: 337-349.

Chen, X., G. Polleichtner, I. Kadurin, and S. Gründer. (2007). Zebrafish Acid-sensing Ion Channel (ASIC) 4, Characterization of Homo- and Heteromeric Channels, and Identification of Regions Important for Activation by H+. J. Biol. Chem. 282(42): 30406-30413.

Chen, Z., G. Kuenze, J. Meiler, and C.M. Canessa. (2021). An arginine residue in the outer segment of hASIC1a TM1 affects both proton affinity and channel desensitization. J Gen Physiol 153:.

Coscoy, S., J.R. de Weille, E. Lingueglia, and M. Lazdunski. (1999). The pre-transmembrane 1 domain of acid-sensing ion channels participates in the ion pore. J. Biol. Chem. 274: 10129-10132.

Couch, T., K. Berger, D.L. Kneisley, T.W. McCullock, P. Kammermeier, and D.M. Maclean. (2021). Topography and motion of acid-sensing ion channel intracellular domains. Elife 10:.

Darboux, I., E. Lingueglia, G. Champigny, S. Coscoy, and P. Barbry. (1998). dGNaC1, a gonad-specific amiloride-sensitive Na+ channel. J. Biol. Chem. 273: 9424-9429.

Della Vecchia, M.C., A.C. Rued, and M.D. Carattino. (2013). Gating Transitions in the Palm Domain of ASIC1a. J. Biol. Chem. 288: 5487-5495.

Deval, E., J. Noël, N. Lay, A. Alloui, S. Diochot, V. Friend, M. Jodar, M. Lazdunski, and E. Lingueglia. (2008). ASIC3, a sensor of acidic and primary inflammatory pain. EMBO. J. 27: 3047-3055.

Durrnagel S., Kuhn A., Tsiairis CD., Williamson M., Kalbacher H., Grimmelikhuijzen CJ., Holstein TW. and Grunder S. (2010). Three homologous subunits form a high affinity peptide-gated ion channel in Hydra. J Biol Chem. 285(16):11958-65.

Edelheit, O., R. Ben-Shahar, N. Dascal, A. Hanukoglu, and I. Hanukoglu. (2014). Conserved charged residues at the surface and interface of epithelial sodium channel subunits--roles in cell surface expression and the sodium self-inhibition response. FEBS J. 281: 2097-2111.

Enuka, Y., I. Hanukoglu, O. Edelheit, H. Vaknine, and A. Hanukoglu. (2012). Epithelial sodium channels (ENaC) are uniformly distributed on motile cilia in the oviduct and the respiratory airways. Histochem Cell Biol 137: 339-353.

Faria, D., N. Lentze, J. Almaça, S. Luz, L. Alessio, Y. Tian, J.P. Martins, P. Cruz, R. Schreiber, M. Rezwan, C.M. Farinha, D. Auerbach, M.D. Amaral, and K. Kunzelmann. (2012). Regulation of ENaC biogenesis by the stress response protein SERP1. Pflugers Arch 463: 819-827.

Fechner, S., I. D''Alessandro, L. Wang, C. Tower, L. Tao, and M.B. Goodman. (2021). DEG/ENaC/ASIC channels vary in their sensitivity to anti-hypertensive and non-steroidal anti-inflammatory drugs. J Gen Physiol 153:.

Finol-Urdaneta, R.K., J.R. McArthur, A. Aboelela, R.S. Bujaroski, H. Majed, A. Rangel, D.J. Adams, M. Ranson, M.J. Kelso, and B.J. Buckley. (2023). Automated Patch Clamp Screening of Amiloride and 5-,-Hexamethyleneamiloride Analogs Identifies 6-Iodoamiloride as a Potent Acid-Sensing Ion Channel Inhibitor. Mol Pharm 20: 3367-3379.

Firsov, D., I. Gautschi, A.-M. Merillat, B.C. Rossier, and L. Schild. (1998). The heterotetrameric architecture of the epithelial sodium channel (ENaC). EMBO J. 17: 344-352.

Fujimoto, A., Y. Kodani, and Y. Furukawa. (2017). Modulation of the FMRFamide-gated Na+ channel by external Ca(2). Pflugers Arch. [Epub: Ahead of Print]

García-Añoveros, J., J.A. García, J.D. Liu, and D.P. Corey. (1998). The nematode degenerin UNC-105 forms ion channels that are activated by degeneration- or hypercontraction-causing mutations. Neuron 20: 1231-1241.

Garty, H. and L.G. Palmer. (1997). Epithelial sodium channels – function, structure, and regulation. Physiol. Rev. 77: 359-396.

Giraldez, T., P. Rojas, J. Jou, C. Flores, and D. Alvarez de la Rosa. (2012). The epithelial sodium channel δ-subunit: new notes for an old song. Am. J. Physiol. Renal Physiol 303: F328-338.

Golubovic, A., A. Kuhn, M. Williamson, H. Kalbacher, T.W. Holstein, C.J. Grimmelikhuijzen, and S. Gründer. (2007). A peptide-gated ion channel from the freshwater polyp Hydra. J. Biol. Chem. 282: 35098-35103.

Gonzales, E.B., T. Kawate, and E. Gouaux. (2009). Pore architecture and ion sites in acid-sensing ion channels and P2X receptors. Nature 460: 599-604.

Hanukoglu, I. (2016). ASIC and ENaC type sodium channels: Conformational states and the structures of the ion selectivity filters. FEBS J. [Epub: Ahead of Print]

Hanukoglu, I. and A. Hanukoglu. (2016). Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases. Gene 579: 95-132.

Henry, P.C., V. Kanelis, M.C. O'Brien, B. Kim, I. Gautschi, J. Forman-Kay, L. Schild, and D. Rotin. (2003). Affinity and specificity of interactions between Nedd4 isoforms and the epithelial Na+ channel. J. Biol. Chem. 278: 20019-20028.

Horisberger, J.-D. (1998). Amiloride-sensitive Na channels. Curr. Opin. Struc. Biol. 10: 443-449.

Huang, L., T. Zou, W. Liang, C. Mo, J. Wei, Y. Deng, and M. Ou. (2023). High-Throughput Sequencing Reveals That Inhibits Colorectal Cancer by Regulating Prognosis-Related Genes. J Pers Med 13:.

Ilyaskin, A.V., F. Sure, V. Nesterov, S. Haerteis, and C. Korbmacher. (2019). Bile acids inhibit human purinergic receptor P2X4 in a heterologous expression system. J Gen Physiol. [Epub: Ahead of Print]

Jasti, J., H. Furukawa, E.B. Gonzales, and E. Gouaux. (2007). Structure of acid-sensing ion channel 1 at 1.9 Å resolution and low pH. Nature 449: 316-323.

Kasimova, M.A., T. Lynagh, Z.P. Sheikh, D. Granata, C.B. Borg, V. Carnevale, and S.A. Pless. (2019). Evolutionarily Conserved Interactions within the Pore Domain of Acid-Sensing Ion Channels. Biophys. J. [Epub: Ahead of Print]

Khan, M.S., N.S. Dosoky, B.K. Berdiev, and J.D. Williams. (2016). Electrochemical impedance spectroscopy for black lipid membranes fused with channel protein supported on solid-state nanopore. Eur Biophys. J. 45: 843-852.

Klipp, R.C., M.M. Cullinan, and J.R. Bankston. (2020). Insights into the molecular mechanisms underlying the inhibition of acid-sensing ion channel 3 gating by stomatin. J Gen Physiol 152:.

Kodani Y. and Furukawa Y. (2014). Electrostatic charge at position 552 affects the activation and permeation of FMRFamide-gated Na+ channels. J Physiol Sci. 64(2):141-50.

Kodani, Y. and Y. Furukawa. (2010). Position 552 in a FMRFamide-gated Na+ channel affects the gating properties and the potency of FMRFamide. Zoolog Sci 27: 440-448.

Konstas, A.A., L.M. Shearwin-Whyatt, A.B. Fotia, B. Degger, D. Riccardi, D.I. Cook, C. Korbmacher, and S. Kumar. (2002). Regulation of the epithelial sodium channel by N4WBP5A, a novel Nedd4/Nedd4-2-interacting protein. J. Biol. Chem. 277: 29406-29416.

Krauson, A.J. and M.D. Carattino. (2016). Thumb domain mediates acid-sensing ion channel desensitization. J. Biol. Chem. [Epub: Ahead of Print]

Kweon, H.J., D.I. Kim, Y. Bae, J.Y. Park, and B.C. Suh. (2016). Acid-Sensing Ion Channel 2a (ASIC2a) Promotes Surface Trafficking of ASIC2b via Heteromeric Assembly. Sci Rep 6: 30684.

Lawong, R.Y., F. May, E.C. Etang, P. Vorrat, J. George, J. Weder, D. Kockler, M. Preller, and M. Althaus. (2023). Recording Sodium Self-Inhibition of Epithelial Sodium Channels Using Automated Electrophysiology in Oocytes. Membranes (Basel) 13:.

Le, T. and M.H. Saier, Jr. (1996). Phylogenetic characterization of the epithelial Na+ channel (ENaC) family. Mol. Membr. Biol. 13: 149-157.

Lee, I.H., A. Dinudom, A. Sanchez-Perez, S. Kumar, and D.I. Cook. (2007). Akt Mediates the Effect of Insulin on Epithelial Sodium Channels by Inhibiting Nedd4-2. J. Biol. Chem. 282(41):29866-29873.

Lee, J.S., H.J. Kweon, H. Lee, and B.C. Suh. (2019). Rapid resensitization of ASIC2a is conferred by three amino acid residues in the N terminus. J Gen Physiol. [Epub: Ahead of Print]

Lemmens-Gruber, R. and S. Tzotzos. (2023). The Epithelial Sodium Channel-An Underestimated Drug Target. Int J Mol Sci 24:.

Lestari, T.F.H., R. Setiyono, N. Tristina, Y. Sofiatin, and Y.W. Hartati. (2023). The optimization of electrochemical immunosensors to detect epithelial sodium channel as a biomarker of hypertension. ADMET DMPK 11: 211-226.

Li, T., Y. Yang, and C.M. Canessa. (2011). Outlines of the pore in open and closed conformations describe the gating mechanism of ASIC1. Nat Commun 2: 399.

Madaio, M.P., I. Czikora, N. Kvirkvelia, M. McMenamin, Q. Yue, T. Liu, H.A. Toque, S. Sridhar, K. Covington, R. Alaisami, P.M. O''Connor, R.W. Caldwell, J.K. Chen, M. Clauss, M.W. Brands, D.C. Eaton, M.J. Romero, and R. Lucas. (2019). The TNF-derived TIP peptide activates the epithelial sodium channel and ameliorates experimental nephrotoxic serum nephritis. Kidney Int 95: 1359-1372.

Mano, I. and M. Driscoll. (1999). DEG/ENaC channels: a touchy superfamily that watches its salts. BioEssays 21: 568-578.

Martin-Malpartida, P., S. Arrastia-Casado, J. Farrera-Sinfreu, R. Lucas, H. Fischer, B. Fischer, D.C. Eaton, S. Tzotzos, and M.J. Macias. (2022). Conformational ensemble of the TNF-derived peptide solnatide in solution. Comput Struct Biotechnol J 20: 2082-2090.

Matalon, S. and H. O’Brodovich. (1999). Sodium channels in alveolar epithelial cells: molecular characterization, biophysical properties, and physiological significance. Annu. Rev. Physiol. 61: 627-661.

Matthewman, C., C.K. Johnson, D.M. Miller Iii, and L. Bianchi. (2018). Functional features of the "finger" domain of DEG/ENaC channels MEC-4 and UNC-8. Am. J. Physiol. Cell Physiol. [Epub: Ahead of Print]

McCleskey, E.W. and M.S. Gold. (1999). Ion channels of nociception. Annu. Rev. Physiol. 61: 835-856.

Mueller, G.M., A.B. Maarouf, C.L. Kinlough, N. Sheng, O.B. Kashlan, S. Okumura, S. Luthy, T.R. Kleyman, and R.P. Hughey. (2010). Cys palmitoylation of the beta subunit modulates gating of the epithelial sodium channel. J. Biol. Chem. 285: 30453-30462.

Noreng, S., A. Bharadwaj, R. Posert, C. Yoshioka, and I. Baconguis. (2018). Structure of the human epithelial sodium channel by cryo-electron microscopy. Elife 7:.

Pao, A.C. (2012). SGK regulation of renal sodium transport. Curr Opin Nephrol Hypertens 21: 534-540.

Price, M.P., G.R. Lewin, S.L. McIlwrath, C. Cheng, J. Xie, P.A. Heppenstall, C.L. Stucky, A.G. Mannsfeldt, T.J. Brennan, H.A. Drummond, J. Qiao, C.J. Benson, D.E. Tarr, R.F. Hrstka, B. Yang, R.A. Williamson, and M.J. Welsh. (2000). The mammalian sodium channel BNC1 is required for normal touch sensation. Nature 407: 1007-1010.

Royal, D.C., L. Bianchi, M.A. Royal, M. Lizzio, Jr, G. Mukherjee, Y.O. Nunez, and M. Driscoll. (2005). Temperature-sensitive mutant of the Caenorhabditis elegans neurotoxic MEC-4(d) DEG/ENaC channel identifies a site required for trafficking or surface maintenance. J. Biol. Chem. 280: 41976-41986.

Saha, B., W. Shabbir, E. Takagi, X.P. Duan, D.C. Almeida Leite Dellova, J. Demko, A. Manis, D. Loffing-Cueni, J. Loffing, M.V. Sørensen, W.H. Wang, and D. Pearce. (2023). Potassium Activates mTORC2-dependent SGK1 Phosphorylation to Stimulate ENaC: Role in Rapid Renal Responses to Dietary Potassium. J Am Soc Nephrol. [Epub: Ahead of Print]

Salinas Castellanos, L.C., R.G. Gatto, S.A. Menchón, M. Blaustein, O.D. Uchitel, and C. Weissmann. (2022). Dynamic Distribution of ASIC1a Channels and Other Proteins within Cells Detected through Fractionation. Membranes (Basel) 12:.

Salinas, M., L.D. Rash, A. Baron, G. Lambeau, P. Escoubas, and M. Lazdunski. (2006). The receptor site of the spider toxin PcTx1 on the proton-gated cation channel ASIC1a. J. Physiol. 570: 339-354.

Sassi, A., Y. Wang, A. Chassot, O. Komarynets, I. Roth, V. Olivier, G. Crambert, E. Dizin, E. Boscardin, E. Hummler, and E. Feraille. (2020). Interaction between Epithelial Sodium Channel -Subunit and Claudin-8 Modulates Paracellular Sodium Permeability in Renal Collecting Duct. J Am Soc Nephrol. [Epub: Ahead of Print]

Saugstad, J.A., J.A. Roberts, J. Dong, S. Zeitouni, and R.J. Evans. (2004). Analysis of the membrane topology of the acid-sensing ion channel 2a. J. Biol. Chem. 279: 55514-55519.

Saxena, S.K., M. Singh, S. Kaur, and C. George. (2006). Distinct domain-dependent effect of syntaxin1A on amiloride-sensitive sodium channel (ENaC) currents in HT-29 colonic epithelial cells. Int J Biol Sci 3: 47-56.

Schaefer, L., H. Sakai, M. Mattei, M. Lazdunski, and E. Lingueglia. (2000). Molecular cloning, functional expression and chromosomal localization of an amiloride-sensitive Na+ channel from human small intestine. FEBS Lett. 471: 205-210.

Schmidt, A., D. Löhrer, R.J. Alsop, P. Lenzig, A. Oslender-Bujotzek, M. Wirtz, M.C. Rheinstädter, S. Gründer, and D. Wiemuth. (2016). A cytosolic amphiphilic alpha helix controls the activity of the bile acid-sensitive ion channel BASIC. J. Biol. Chem. [Epub: Ahead of Print]

Schuhmacher LN., Srivats S. and Smith ES. (2015). Structural domains underlying the activation of acid-sensing ion channel 2a. Mol Pharmacol. 87(4):561-71.

Scott, D.W., M.P. Walker, J. Sesma, B. Wu, T.J. Stuhlmiller, J.R. Sabater, W.M. Abraham, T.M. Crowder, D.J. Christensen, and R. Tarran. (2017). SPX-101 Is a Novel Epithelial Sodium Channel-targeted Therapeutic for Cystic Fibrosis That Restores Mucus Transport. Am J Respir Crit Care Med 196: 734-744.

Sedensky, M.M., J.M. Siefker, J.Y. Koh, D.M. Miller, 3rd, and P.G. Morgan. (2004). A stomatin and a degenerin interact in lipid rafts of the nervous system of Caenorhabditis elegans. Am. J. Physiol. Cell Physiol. 287: C468-474.

Sheikh, Z.P., M. Wulf, S. Friis, M. Althaus, T. Lynagh, and S.A. Pless. (2021). The M1 and pre-M1 segments contribute differently to ion selectivity in ASICs and ENaCs. J Gen Physiol 153:.

Sheng, S., J. Li, K.A. McNulty, D. Avery, and T.R. Kleyman. (2000). Characterization of the selectivity filter of the epithelial sodium channel. J. Biol. Chem. 275: 8572-8581.

Sheng, S., J. Li, K.A. McNulty, T. Kieber-Emmons, and T.R. Kleyman. (2001a). Epithelial sodium channel pore region: structure and role in gating. J. Biol. Chem. 276: 1326-1334.

Sheng, S., K.A. McNulty, J.M. Harvey, and T.R. Kleyman. (2001b). Second transmembrane domains of ENaC subunits contribute to ion permeation and selectivity. J. Biol. Chem. 276: 44091-44098.

Shi S. and Kleyman TR. (2013). Gamma subunit second transmembrane domain contributes to epithelial sodium channel gating and amiloride block. Am J Physiol Renal Physiol. 305(11):F1585-92.

Shi, S., C.J. Luke, M.T. Miedel, G.A. Silverman, and T.R. Kleyman. (2016). Activation of the Caenorhabditis elegans degenerin channel by shear stress requires the MEC-10 subunit. J. Biol. Chem. [Epub: Ahead of Print]

Shi, S., D.D. Ghosh, S. Okumura, M.D. Carattino, O.B. Kashlan, S. Sheng, and T.R. Kleyman. (2011). Base of the thumb domain modulates epithelial sodium channel gating. J. Biol. Chem. 286: 14753-14761.

Shi, S., S.M. Mutchler, B.M. Blobner, O.B. Kashlan, and T.R. Kleyman. (2018). Pore-lining residues of MEC-4 and MEC-10 channel subunits tune the degenerin channel''s response to shear stress. J. Biol. Chem. [Epub: Ahead of Print]

Snyder, P.M., D.R. Olson, F.J. McDonald, and D.B. Bucher. (2001). Multiple WW domains, but not the C2 domain, are required for inhibition of the epithelial Na+ channel by human Nedd4. J. Biol. Chem. 276: 28321-28326.

Song, N., Z. Lu, J. Zhang, Y. Shi, Y. Ning, J. Chen, S. Jin, B. Shen, Y. Fang, J. Zou, J. Teng, X.P. Chu, L. Shen, and X. Ding. (2019). Acid-sensing ion channel 1a is involved in ischaemia/reperfusion induced kidney injury by increasing renal epithelia cell apoptosis. J Cell Mol Med. [Epub: Ahead of Print]

Springauf, A., P. Bresenitz, and S. Gründer. (2011). The interaction between two extracellular linker regions controls sustained opening of acid-sensing ion channel 1. J. Biol. Chem. 286: 24374-24384.

Su, X., Q. Li, K. Shrestha, E. Cormet-Boyaka, L. Chen, P.R. Smith, E.J. Sorscher, D.J. Benos, S. Matalon, and H.L. Ji. (2006). Interregulation of proton-gated Na+ channel 3 and cystic fibrosis transmembrane conductance regulator. J. Biol. Chem. 281: 36960-36968.

Sun, D., S. Liu, S. Li, M. Zhang, F. Yang, M. Wen, P. Shi, T. Wang, M. Pan, S. Chang, X. Zhang, L. Zhang, C. Tian, and L. Liu. (2020). Structural insights into human acid-sensing ion channel 1a inhibition by snake toxin mambalgin1. Elife 9:.

Sun, H.W., X.P. Chu, R.P. Simon, Z.G. Xiong, and T.D. Leng. (2023). Inhibition of Acid-Sensing Ion Channels by KB-R7943, a Reverse Na/Ca Exchanger Inhibitor. Biomolecules 13:.

Takeda, A.N., I. Gautschi, M.X. van Bemmelen, and L. Schild. (2007). Cadmium trapping in an epithelial sodium channel pore mutant. J. Biol. Chem. 282: 31928-31936.

Tolino, L.A., S. Okumura, O.B. Kashlan, and M.D. Carattino. (2011). Insights into the mechanism of pore opening of acid-sensing ion channel 1a. J. Biol. Chem. 286: 16297-16307.

Ugawa, S., Y. Ishida, T. Ueda, K. Inoue, M. Nagao, and S. Shimada. (2007). Nafamostat mesilate reversibly blocks acid-sensing ion channel currents. Biochem. Biophys. Res. Commun. 363: 203-208.

Ugawa, S., Y. Ishida, T. Ueda, Y. Yu, and S. Shimada. (2008). Hypotonic stimuli enhance proton-gated currents of acid-sensing ion channel-1b. Biochem. Biophys. Res. Commun. 367: 530-534.

van Bemmelen, M.X., D. Huser, I. Gautschi, and L. Schild. (2015). The Human Acid-Sensing Ion Channel ASIC1a: Evidence for a Homotetrameric Assembly State at the Cell Surface. PLoS One 10: e0135191.

Vullo, S., N. Ambrosio, J.P. Kucera, O. Bignucolo, and S. Kellenberger. (2021). Kinetic analysis of ASIC1a delineates conformational signaling from proton-sensing domains to the channel gate. Elife 10:.

Waldmann, R., G. Champigny, F. Bassilana, C. Heurteaux, and M. Lazdunski. (1997). A proton-gated cation channel involved in acid-sensing. Nature 386: 173-177.

Wang, W., B. Duan, H. Xu, L. Xu, and T.-L. Xu. (2006). Calcium-permeable acid-sensing ion channel is a molecular target of the neurotoxic metal ion lead. J. Biol. Chem. 281: 2497-2505.

Wang, Y., A. Apicella, Jr, S.K. Lee, M. Ezcurra, R.D. Slone, M. Goldmit, W.R. Schafer, S. Shaham, M. Driscoll, and L. Bianchi. (2008). A glial DEG/ENaC channel functions with neuronal channel DEG-1 to mediate specific sensory functions in C. elegans. EMBO. J. 27: 2388-2399.

Wang, Y., H. Zhou, Y. Sun, and Y. Huang. (2022). Acid-sensing ion channel 1: potential therapeutic target for tumor. Biomed Pharmacother 155: 113835.

Welsh, M.J., M.P. Price, and J. Xie. (2002). Biochemical basis of touch perception: mechanosensory function of degenerin/epithelial Na+ channels. J. Biol. Chem. 277: 2369-2372.

Wiemuth, D. and S. Gründer. (2010). A single amino acid tunes Ca2+ inhibition of brain liver intestine Na+ channel (BLINaC). J. Biol. Chem. 285: 30404-30410.

Yang, L. and L.G. Palmer. (2018). Determinants of selective ion permeation in the epithelial Na channel. J Gen Physiol. [Epub: Ahead of Print]

Yao, Z., L. Yuan, X. Chen, Q. Wang, L. Chai, X. Lu, F. Yang, Y. Wang, and S. Yang. (2023). A thermal receptor for nonvisual sunlight detection in myriapods. Proc. Natl. Acad. Sci. USA 120: e2218948120.

Zhao, R., X. Liang, M. Zhao, S.L. Liu, Y. Huang, S. Idell, X. Li, and H.L. Ji. (2014). Correlation of apical fluid-regulating channel proteins with lung function in human COPD lungs. PLoS One 9: e109725.

Zhong, L., R.Y. Hwang, and W.D. Tracey. (2010). Pickpocket is a DEG/ENaC protein required for mechanical nociception in Drosophila larvae. Curr. Biol. 20: 429-434.

Examples:

TC#NameOrganismal TypeExample
1.A.6.1.1

Epithelial Na+ channel, ENaC (regulates salt and fluid homeostasis and blood pressure; regulated by Nedd4 isoforms and SGK1, 2 and 3 kinases) (Henry et al., 2003; Pao 2012).  Cd2+ inhibits α-ENaC by binding to the internal pore where it interacts with residues in TMS2 (Takeda et al., 2007).  The channel is regulated by palmitoylation of the beta subunit which modulates gating (Mueller et al. 2010). ENaCs are more selective for Naa+ over other cations than ASICs (Yang and Palmer 2018). ENaC plays a role in chronic obstructive pulmonary diseases (COPD) (Zhao et al. 2014). The hetrodimeric complex can consist of αβγ or δβγ subunits, depending on the tissue (Giraldez et al. 2012).  The α- and γ-subunits of the epithelial Na+ channel interact directly with the Na+:Cl- cotransporter, NCC, in the renal distal tubule with functional cosequences, and together they determine bodily salt balance and blood pressure (Mistry et al. 2016).  ENaC is regulated by syntaxins (Saxena et al. 2006). The cryoEM structure has been solved (Noreng et al. 2018). Interactions between the epithelial sodium channel gamma-subunit and claudin-8 modulates paracellular sodium permeability in the renal collecting duct (Sassi et al. 2020). Tumer necrosis factor, TNF, of 233 aas, is the source of a modified cyclic peptide of 17 aas, solnatide or the TIP peptide, (CGQRETPEGAEAKPWYC), residues 177 - 195), that activates ENaC (Madaio et al. 2019; Martin-Malpartida et al. 2022). Acid-Sensing ion channels are inhibited by KB-R7943, a reverse Na+/Ca2+ exchanger (see TC# 1.D.208) (Sun et al. 2023).

Animals

αβγ- or δβγ-ENaC heterotrimeric epithelial Na+ channel of Homo sapiens

 
1.A.6.1.10

Acid-sensing ion channel 1, ACCN2 of 514 aas and 2 TMSs.

ACCN2 of Lampetra fluviatilis (European river lamprey) (Petromyzon fluviatilis)

 
1.A.6.1.11

(Bile) acid-sensitive ion channel, BASIC (ASIC, ACCN5, HINAC), of 505 aas. Cation channel that gives rise to very low constitutive currents in the absence of activation. The activated channel exhibits selectivity for sodium, and is inhibited by amiloride (Schaefer et al. 2000).  A cytoplasmic amphipathic α-helix controls activity (Schmidt et al. 2016). This system may be present in mitochondria ().

 

BASIC of Homo sapiens

 
1.A.6.1.12

Duplicated ENaC with 990 aas and 4 TMSs in a 1 + 2 + 1 TMS arrangement.

Duplicated ENaC of Exaiptasia pallida

 
1.A.6.1.13

Acid-sensing ion channel 5 isoform X1 pf 639 aas and possibly 7 TMSs with 5 TMSs in an N-terminal domain not related to ASICs followed by two TMSs, one N-terminal and one C-terminal, all in the ASIC domain of the protein.

ASIC5 of Brachionus plicatilis

 
1.A.6.1.14

FMRFamide (peptide)-gated ionotropic receptor Na+ channel, NaC2-4 or NaC2, 3 and 5 (gated by neuropeptides Hydra-RFamides I and II; present in tentacles) (Golubovic et al. 2007). Three homologous subunits, NaC2, 3 and 5, assemble to form a more typical high affinity peptide-gated ion channel (Durrnagel et al., 2010).

Cniderians

NaC2-5 of Hydra magnipapillata:
NaC2 - A8DZR6
NaC3 - A8DZR7
NaC4 - A8DZR8
NaC5 - D3UD58

 
1.A.6.1.15

Uncharacterized protein of 1029 aas and 4 TMSs, two near the N- and C-termini, and two more at residues 410 and 600. The region of homology with other members of the family are residues 570 to 1000, thus including the last two TMSs.

UP of Cloeon dipterum

 
1.A.6.1.16

Uncharacterized protein of 418 aas and 3 TMSs.

UP of Allacma fusca

 
1.A.6.1.17

Broad-range thermal receptor 1 protein of 431 aas and 2 TMSs, N- and C-terminal. A nonvisual and extraocular sunlight detection mechanism occurs via the broad-range thermal receptor 1 (BRTNaC1, temperature range = 33 to 48 °C) in centipede antennae. BRTNaC1, a heat-activated cation-permeable ion channel, Heat activation of BRTNaC1 exhibits strong pH dependence controlled by two protonatable sites. Physiologically, temperature-dependent activation of BRTNaC1 upon sunlight exposure comes from a striking photothermal effect on the antennae, where a slightly acidic environment (pH 6.1) of the body fluid leads to the protonation of BRTNaC1 and switches on its high thermal sensitivity. Testosterone potently inhibits heat activation of BRTNaC1 and the sunlight avoidance behavior of centipedes. This suggests a sophisticated strategy for nonvisual sunlight detection in myriapods (Yao et al. 2023).

BRTNaCl of Scolopendra subspinipes (centipede)

 
1.A.6.1.18

Acid-sensing ion channel 1B-like of 524 aas and 3 TMSs.

ASIC 1B of Daphnia pulex

 
1.A.6.1.2

Amiloride-sensitive cation channel, ASIC1/ASIC3 (also called ASIC1a, BNC1, MDEG, ACCN2 and BNAC2), which is an acid-sensitive (proton-gated) homo- or hetero-oligomeric cation (Na+ (high affinity), Ca2+, K+) channel. It it 98% identical to the human ortholog and associates with DRASIC tomediate touch sensation, being a mechanosensor (lead inhibited) channel (Wang et al., 2006). In pulmonary tissue (lung epithelial cells) it and CFTR interregulate each other (Su et al., 2006). ASIC3 is a sensor of acidic and primary inflammatory pain (Deval et al., 2008).  Acid sensing ion channel-1b (ASIC1b), virtually identical to  the rat and human orthologs, is stimulated by hypotonic stimuli  (Ugawa et al., 2007; Deval et al., 2008). This protein is 98% idientical to the human ortholog Z(as noted above), which is an excitatory neuronal cation channel, involved in physiopathological processes related to extracellular pH fluctuation such as nociception, ischaemia, perception of sour taste and synaptic transmission. The spider peptide toxin psalmotoxin 1 (PcTx1) inhibits its proton-gated cation channel activity (Salinas et al. 2006). ASIC1a localizes to the proximal tubular and contributes to ischaemia/reperfusion (I/)R induced kidney injury (Song et al. 2019). Stomatin (STOM; TC# 8.A.21.1.1) is an inhibitor of ASIC3, and it is anchored to the ASIC3 channel via a site on the distal C-terminus of the channel to stabilizes the desensitized state  via an interaction with TMS1 (Klipp et al. 2020). Sun et al. 2020 presented single-particle cryo-EM structures of human ASIC1a (hASIC1a) and the hASIC1a-Mamba1 complex at resolutions of 3.56 and 3.90 Å, respectively. The structures revealed the inhibited conformation of hASIC1a upon Mamba1 binding. Mamba1 prefers to bind hASIC1a in a closed state and reduces the proton sensitivity of the channel, representing a closed-state trapping mechanism. Kinetic analyses of ASIC1a delineated conformational signaling from proton-sensing domains to the channel gate (Vullo et al. 2021). An arginine residue in the outer segment of hASIC1a TMS1 affects both proton affinity and channel desensitization (Chen et al. 2021). Acid-sensing ion channels (ASICs) are weakly sodium selective (sodium:potassium ratio approximately 10:1), while ENaCs show a high preference for sodium over potassium (>500:1). The pre-TMS1 and TMS1 regions of mASIC1a channels are major determinants of ion selectivity (Sheikh et al. 2021). ASIC1a shuttles between the membranous organellar fraction to the plasm membrane (Salinas Castellanos et al. 2022). Multiscale molecular dynamics simulations predict arachidonic acid binding sites in human ASIC1a and ASIC3 transmembrane domains (Ananchenko and Musgaard 2023). Rotundine inhibits the development and progression of colorectal cancer by regulating the expression of prognosis-related genes such as ASIC3 (ACCN3; SLNAC1, TNACT) in humans (Huang et al. 2023).

Animals

αβγENaC of Rattus norvegicus.
DRASIC (O35240)
ASIC3 (O55163)
ASIC1 (P55926)

 
1.A.6.1.3

The epithelial Na+ channel, EnaC5 (involved in fluid and electrolyte homeostasis). The C-terminus of each subunit (α, β, and γ) contains a PPXY motif for interaction with the WW domains of the ubiquitin-protein ligases, Nedd4 and Nedd4-2. Disruption of this interaction, as in Liddle's syndrome where mutations delete or alter the PPXY motif of either the β or γ subunits, has been shown to result in increased ENaC activity and arterial hypertension. N4WBP5A (Nedd4-family interacting protein-2) plays a role (see 8.A.30; Konstas et al., 2002). Wiemuth & Grunder (2010) showed that an unknown ligand, interacting with an amino acyl residue in the extracellular domain, tunes Ca2+ inhibition in the rat protein, but not the mouse orthologue.

Animals

ENaC5 of Rattus norvegicus (Q9R0W5)

 
1.A.6.1.4ACD-1 (degenerin-like glial acid-sensitive channel) is constitutively open and impermeable to Ca2+, yet is required with neuronal DEG/ENaC channel, DEG-1 (1.A.6.2.1) for acid avoidance and chemotaxis to the amino acid lysine (Wang et al. 2008).

animal

ACD-1 of Caenorhabditis elegans (P91102)

 
1.A.6.1.5

Neuronal acid-sensing cation channel-1, ASIC1 (>90% identical to ASIC1 of Rat (TC#1.A.6.1.2)). 3D structure (1.9Å resolution) has been solved (Jasti et al., 2007). Regulated by the glucocorticoid-induced kinase-1 isoform 1 (SGK1.1) (Arteaga et al., 2008). Residues in the second transmembrane domain of the ASIC1a that contribute to ion selectivity have been defined (Carattino and Della Vecchia, 2012). Outlines of the pore in open and closed conformations describe the gating mechanism (Li et al., 2011). Interactions between two extracellular linker regions control sustained channel opening (Springauf et al., 2011).  Can form monomers, trimers and tetramers, but the tetramer may be the predominant species in the plasma membrane (van Bemmelen et al. 2015). The C-terminal tail projects into the cytosol by approximately 35 Å, and the N and C tails from the same subunits are closer than those of adjacent subunits (Couch et al. 2021).

Animals

ASIC-1 of Gallus gallus (Q1XA76)

 
1.A.6.1.6Acid sensing cation channel ASIC4.1 (senses and gated by extracellular pH) (forms homomers and heteromers with ASIC4.2) (Chen et al., 2007)AnimalsASIC4.1 of Danio rerio (Q708S4)
 
1.A.6.1.7Acid sensing cation channel ASIC4.2 (does not sense extracellular pH) (forms homomers and heteromers with ASIC4.1) (Chen et al., 2007).AnimalsASIC4.2 of Danio rerio (Q708S3)
 
1.A.6.1.8

Amiloride and acid-sensitive cation channels, ASIC2a and ASIC2b are splice variants of the same gene (ACCN1, ACCN, BNAC1, MDEG) product.  Regions involved in acid (proton) sensing and confering tachyphylasis have been identified (Schuhmacher et al. 2015).  ASIC2 isoforms have different subcellular distributions: ASIC2a targets the cell surface while ASIC2b resides in the ER. TMS1 and the proximal post-TMS1 domain (17 amino acids) of ASIC2a are critical for membrane targeting, and replacement of corresponding residues in ASIC2b by those of ASIC2a conferred proton-sensitivity as well as surface expression to ASIC2b (Kweon et al. 2016). This protein is 99% identical to the human ortholog with acc# Q16515.  Rapid resensitization of ASIC2a is conferred by three amino acid residues near the N terminus (Lee et al. 2019). The human ortholog of ASIC1 (UniProt acc # P783480 is 98% identical to the mouse ortholog. ASIC1 plays a role in the occurrence and development of several types of tumors (Wang et al. 2022).

Animals

ASIC1b of Mus musculus

 
1.A.6.1.9

Acid-sensing ion channel 2, ASIC2, of 520 aas and 2 TMSs.

ASIC2 of Petromyzon marinus (Sea lamprey)

 
Examples:

TC#NameOrganismal TypeExample
1.A.6.2.1Degenerin-1 Worm Degenerin-1 of Caenorhabditis elegans (P24585)
 
1.A.6.2.2

Touch-responsive mechanosensitive degenerin channel complex (Mec-4/Mec-10 form the cation/Ca2+-permeable channel; Mec-2 and Mec-6 regulate) (Bianchi, 2007; Chelur et al., 2002; ). Mec-6 is a chaparone protein required for functional insertion (Matthewman et al. 2018). Mec-10 plays a role in the response to mechanical forces such as laminar shear stress (Shi et al. 2016). MEC-4 or MEC-10 mutants that alter the channel's LSS response are primarily clustered between the degenerin site and the selectivity filter, a region that likely forms the narrowest portion of the channel pore (Shi et al. 2018). TMS2 forms the Ca2+ channel of Mec-4.  A C-terminal domain affects trafficking of a neuronally expressed DEG/ENaC. Neuronal swelling occurs prior to commitment to necrotic death (Royal et al. 2005).

Worm

Mec-2, 4, 6, 10 mechanosensitive degenerin channel complex in Caenorhabditis elegans
Mec-4
Mec-10
Mec-6
Mec-2

 
1.A.6.2.3Degenerin channel, UNC-105. (Activated by degeneration or hypercontraction-causing mutations) (Bianchi, 2007; García-Añoveros et al., 1998)

Animals

UNC-105 of Caenorhabditis elegans (Q09274)

 
1.A.6.2.4

Motility and anesthetic-sensitive degenerin, UNC-8 (Uncoordinated protein-8) Na+ (not Ca2+) channel (regulated by UNC-1 (a mammalian stomatin homologue)). UNC-1 and UNC-8 are found in cholesterol/sphingolipid rafts together with UNC-24 (Bianchi, 2007; Sedensky et al., 2004). UNC-8 is inhibited by μM concentrations of extracellular divalent cations mediated by the extracellular finger domain (Matthewman et al. 2018).

Animals

UNC-8 of Caenorhaditis elegans (Q21974)

 
1.A.6.2.5

Mechanotransduction degenerin, DEL-1 (Bianchi, 2007).

Animals

DEL-1 of Caenorhabditis elegans (Q19038)

 
1.A.6.2.6

Serum paraoxonase/arylesterase 1, PON 1 (Aromatic esterase 1) (A-esterase 1) (Serum aryldialkylphosphatase 1)

Animals

PON1 of Homo sapiens

 
1.A.6.2.7

Ion channel of 686 aas and 2 TMSs, one at the N-terminus and one at the C-terminus.  The N-terminal half of this protein is cycsteine-rich and shows similarity with 9.B.87.1.12, while the C-terminal half shows extensive similarity with 1.A.6.2 proteins.

Ion channel of Pristionchus pacificus

 
Examples:

TC#NameOrganismal TypeExample
1.A.6.3.1Peptide neurotransmitter-gated ionotropic receptor Snail Phe-Met-Arg-Phe-NH2-activated Na+ channel of Helix aspersa
 
1.A.6.3.2

FMRFamide (peptide)-gated  sodium channel, FaNaC.  The charge on aspartate-552 in TMS2 influcences the gating properties and potency of the channel (Kodani and Furukawa 2010; Kodani and Furukawa 2014).  The FMRFamide-evoked current through AkFaNaC was depressed 2-3-fold by millimolar (1.8 mM) Ca2+ (Fujimoto et al. 2017). Both D552 and D556 were indispensable for the sensitivity of FaNaC to millimolar Ca2+. The Ca2+-sensitive gating was recapitulated by an allosteric model in which Ca2+-bound channels are more difficult to open. The desensitization of FaNaC was also inhibited by Ca2+ (Fujimoto et al. 2017).

Animals

FaNaC of Aplysia kurodai

 
1.A.6.3.3

Uncharacterized protein of 577 aas and 2 TMSs, N- and C-terminal.

UP of Taenia asiatica

 
1.A.6.3.4

Uncharacterized protein of 616 aas and 2 TMSs, N- and C-terminal.

UP of Hymenolepis diminuta

 
1.A.6.3.5

Uncharacterized protein of 534 aas and 2 TMSs.

UP of Helobdella robusta

 
Examples:

TC#NameOrganismal TypeExample
1.A.6.4.1

Ripped pocket (Rpk) fly gonad-specific Na+ channel (amiloride-sensitive) (Adams et al., 1998).

Animals

Rpk of Drosophila melanogaster

 
1.A.6.4.2

Pickpocket (Adams et al., 1998; Zhong et al., 2010).

Animals

Pickpocket of Drosophila melanogaster (Q7KT94)

 
1.A.6.4.3

Putative Na+ channel 

Animals

Putative Na+ channel of Drosophila melanogaster (O61365)

 
1.A.6.4.4

Na+ channel protein, NaCh, of 522 aas with 2 or 3 TMSs in a 1 (N-terminal) + 1 or 2 TMSs (C-terminal).

NaCh of Cyphomyrmex costatus

 
1.A.6.4.5

Uncharacterized protein of 509 aas and 2 TMSs, N- and C-terminal.

UP of Laodelphax striatellus (small brown planthopper)

 
1.A.6.4.6

Sodium channel protein Nach-like protein, NaCh, of 533 aas with the usual 2N- and C-terminal TMSs, but possibly as many as 6 smaller peaks of hydrophobicity (TMSs?) in between these two TMSs.

NaCh of Vollenhovia emeryi

 
Examples:

TC#NameOrganismal TypeExample
Examples:

TC#NameOrganismal TypeExample