Epithelial sodium channel (ENaC) in GtoPdb v.2025.3

OverviewThe epithelial sodium channels (ENaC) are located on the apical membrane of epithelial cells in the kidney tubules, lung, respiratory tract, male and female reproductive tracts, sweat and salivary glands, placenta, colon, and some other organs [10, 49, 15, 24, 23]. In these epithelia, Na+ ions flow from the extracellular fluid into the cytoplasm of epithelial cells via ENaC and are then pumped out of the cytoplasm into the interstitial fluid by the Na+/K+ ATPase located on the basolateral membrane [43]. As Na+ is one of the major electrolytes in the extracellular fluid (ECF), osmolarity change initiated by the Na+ flow is accompanied by a flow of water [7]. Thus, ENaC has a central role in regulating ECF volume and blood pressure, primarily via its function in the kidney [44]. The expression of ENaC subunits, hence its activity, is regulated by the renin-angiotensin-aldosterone system, and other factors involved in electrolyte homeostasis [44, 33].The genetics of the hereditary systemic pseudohypoaldosteronism type-I revealed that the activity of ENaC is dependent on three subunits encoded by three genes [24, 12]. Within the protein superfamily that includes ENaC, the crystal structure of ASIC was determined first, revealing a trimeric structure with a large extracellular domain anchored in the membrane with a bundle of six TM helices (two TM helices/subunit) [3, 27]. The first 3D structure of human ENaC was determined by single-particle cryo-electron microscopy at a resolution of 3.7 Å [39]. A recent study improved the resolution to 3 Å [40]. These structures confirmed that ENaC has a 3D quaternary structure similar to ASIC. ENaC is assembled as a hetero-trimer with a clockwise order of α-γ-β subunit viewed from the top, as shown previously [13]. In contrast to ASIC1 which can assemble into a functional homotrimer, ENaC activity can be reconstituted fully only as a heterotrimer with an αβγ or a δβγ composition [30]. In the respiratory tract and female reproductive tract, large segments of the epithelia are composed of multi-ciliated cells. In these cells, ENaC is located along the entire length of the cilia that cover the cell surface [17]. Cilial location greatly increases ENaC density per cell surface and allows ENaC to serve as a sensitive regulator of osmolarity of the periciliary fluid throughout the whole depth of the fluid bathing the cilia [17]. In contrast to ENaC, CFTR (ion transporter defective in cystic fibrosis) is located on the non-cilial cell surface [17]. In the vas deferens segment of the male reproductive tract, the luminal surface is covered by microvilli and stereocilia projections with backbones composed of actin filament bundles [49]. In these cells, both ENaC and the water channel aquaporin AQP9 are localized on these projections and also in the basal and smooth muscle layers [49]. Thus, ENaC function regulates the volume of fluid lining epithelia essential for mucociliary clearance of respiratory airways, transport of germ cells, fertilization, implantation, and cell migration [38, 17, 24]. Genes and PhylogenyIn the human genome, there are four homologous genes (SCNN1A, SCNN1B, SCNN1D, and SCNN1G) that encode four proteins, α-, β-, γ-, and δ-ENaC that may be involved in the assembly of ENaC [11, 35, 48, 54]. These four subunits share 23-34% sequence identity and <20% identity with ASIC subunits [24]. The genes coding for all four ENaC subunits are present in all bony vertebrates with the exception of ray-finned fish genomes that have lost all ENaC genes. The mouse genome has lost the gene SCNN1D that codes for δ-ENaC [19, 24, 24]. The α-, β-, and γ-ENaC genes are also present in jawless vertebrates (e.g., lampreys) and cartilaginous fishes (e.g., sharks) [24]. Examination of the methylation patterns of the 5\u27-flanking region of SCNN1A, SCNN1B, and SCNN1G genes in human cells showed an inverse correlation between gene expression and DNA methylation, suggesting epigenetic transcriptional control of ENaC genes [42]. Channel biogenesis, assembly and functionThe expression of ENaC subunits is regulated primarily by aldosterone and many additional extracellular and intracellular factors [44, 32, 41]. Most of the studies indicate that the expression of the three subunits is not coordinated [9]. However, the transport of the subunits to the membrane is dependent on three intact subunits. Even a missense mutation in one subunit reduces the concentration of assembled channels on the cell surface [16]. ENaC is a constitutively active channel, i.e., the flow of Na+ ions is not dependent on an activating factor. Hence, heterologous cells expressing ENaC (e.g., Xenopus oocytes), must be maintained in a solution that contains amiloride to keep ENaC inhibited. To measure ENaC activity, the bath solution is switched to a solution without amiloride. ENaC has two major states: 1) Open, and 2) Closed. The probability of ENaC being in the open state is called ENaC open probability (Po). ENaC activity is regulated by a diverse array of factors that exert their effects by modifying, directly or indirectly, two major parameters: 1) The density of ENaC in the membrane; and 2) The channel open probability [28, 30]. The Po of ENaC is greatly decreased by external Na+ and this response is called Na+ self-inhibition [50, 4, 26].An important aspect of ENaC regulation is that the α and the γ subunits have conserved serine protease cleavage sites in the extracellular segment [24]. Cleavage of these subunits by proteases such as furin and plasmin leads to the activation of ENaC [45, 31, 1].Diseases associated with ENaC mutationsMutations in any of the three genes (SCNN1A, SCNN1B, and SCNN1G) may cause partial or complete loss of ENaC activity, depending on the mutation [12, 21]. Such loss-of-function mutations are associated with a syndrome named "systemic" or "multi-system" autosomal recessive pseudohypoaldosteronism type I (PHA1B) [20, 12, 24, 17, 56, 47]. So far, no mutation has been found in the SCNN1D gene that causes PHA. PHA patients suffer from severe salt loss from all aldosterone target organs expressing ENaC, including kidney, sweat and salivary glands and respiratory tract. During infancy and early childhood, the severe electrolyte disturbances, dehydration and acidosis may require recurrent hospitalizations. The severity and frequency of salt-wasting episodes improve with age [22]. PHA1B is also associated with a dysfunctional female reproductive system [17, 6]. The carboxy-terminal of ENaC includes a short consensus sequence called the PY motif. Mutations in this motif in SCNN1B and SCNN1G are associated with Liddle syndrome, which is characterized by early-onset hypertension [5, 51]. The PY motif is recognized by Nedd4-2 that is a ubiquitin ligase. Thus, mutations in the PY motif reduce ubiquitylation of ENaC leading to the accumulation of ENaC in the membrane, consequently enhance the activity of ENaC [46].ENaC expression in tumorsThe observation that [Na+] is higher in many cancerous cells as compared to non-cancerous cells has led to the suggestion that enhanced expression of ENaC may be responsible for increased metastasis [34]. However, analysis of RNA sequencing data of ENaC-encoding genes, and clinical data of cervical cancer patients from The Cancer Genome Atlas showed a negative correlation with histologic grades of tumor [52]. Similarly, studies on breast cancer cells that altered α-ENaC levels by over-expression or siRNA-mediated knockdown showed that increased α-ENaC expression was associated with decreased breast cancer cell proliferation [55]. In contrast, analysis of RNA sequencing data from The Cancer Genome Atlas showed that high expression of SCNN1A was correlated with poor prognosis in patients with ovarian cancer [36]. These findings indicate that the association of ENaC levels with tumorigenesis varies depending on the tissue.COVID-19The surface of SARS-CoV-2 virions that cause COVID-19 is covered by many glycosylated S (spike) proteins. These S proteins bind to the membrane-bound angiotensin-converting enzyme 2 (ACE2) as a first step in the entry of the virion into the host cell. Viral entry into the cell is dependent on the cleavage of the S protein (at Arg-667/Ser-668) by a serine-protease. Anand et al. showed that this cleavage site has a sequence motif that is homologous to the furin cleavage site in α-ENaC [2]. A comprehensive review on the pathological consequences of COVID-19 suggests a role for ENaC in the early phases of COVID-19 infection in the respiratory tract epithelia [18]

3.6.5.2 Small monomeric GTPases in GtoPdb v.2025.3

Small G-proteins, are a family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate (GTP). They are a type of G-protein found in the cytosol that are homologous to the alpha subunit of heterotrimeric G-proteins, but unlike the alpha subunit of G proteins, a small GTPase can function independently as a hydrolase enzyme to bind to and hydrolyze a guanosine triphosphate (GTP) to form guanosine diphosphate (GDP). The best-known members are the Ras GTPases and hence they are sometimes called Ras subfamily GTPases

SLC3 and SLC7 families of heteromeric amino acid transporters (HATs) in GtoPdb v.2025.1

The SLC3 and SLC7 families combine to generate functional transporters, where the subunit composition is a disulphide-linked combination of a heavy chain (SLC3 family) with a light chain (SLC7 family) [1]

Type XIII RTKs: Ephrin receptor family in GtoPdb v.2025.3

Ephrin receptors are a family of 15 RTKs - the largest family of RTKs - with two identified subfamilies (EphA and EphB), which have a role in the regulation of neuronal development, cell migration, patterning and angiogenesis. Their ligands are membrane-associated proteins, thought to be glycosylphosphatidylinositol-linked for EphA (ephrin-A1 , ephrin-A2, ephrin-A3, ephrin-A4 and ephrin-A5) and transmembrane proteins for Ephrin B (ENSFM00250000002014: ephrin-B1, ephrin-B2 and ephrin-B3). Ephrin-A3 and ephrin-B3 have also been shown to interact with heparan sulphate proteoglycans [22]

Voltage-gated potassium channels (Kv) in GtoPdb v.2025.3

The 6TM family of K channels comprises the voltage-gated KV subfamilies, the EAG subfamily (which includes hERG channels), the Ca2+-activated Slo subfamily (actually with 7TM, termed BK) and the Ca2+-activated SK subfamily. These channels possess a pore-forming α subunit that comprise tetramers of identical subunits (homomeric) or of different subunits (heteromeric). Heteromeric channels can only be formed within subfamilies (e.g. Kv1.1 with Kv1.2; Kv7.2 with Kv7.3). The pharmacology largely reflects the subunit composition of the functional channel.Kv7 channelsKv7.1-Kv7.5 (KCNQ1-5) K+ channels are voltage-gated K+ channels with major roles in neurons, muscle cells and epithelia where they underlie physiologically important K+ currents, such as the neuronal M-current and the cardiac IKs. Genetic deficiencies in all five KCNQ genes result in human excitability disorders, including epilepsy, autism spectrum disorders, cardiac arrhythmias and deafness. Thanks to the recent knowledge of the structure and function of human KCNQ-encoded proteins, these channels are increasingly used as drug targets for treating diseases [333, 2, 777, 294]

Corticotropin-releasing factor receptors in GtoPdb v.2025.3

Corticotropin-releasing factor (CRF, nomenclature as agreed by the NC-IUPHAR subcommittee on Corticotropin-releasing Factor Receptors [35]) receptors are activated by the endogenous peptides corticotrophin-releasing hormone, a 41 amino-acid peptide, urocortin 1, 40 amino-acids, urocortin 2, 38 amino-acids and urocortin 3, 38 amino-acids. CRF1 and CRF2 receptors are activated non-selectively by CRH and UCN. CRF2 receptors are selectively activated by UCN2 and UCN3. Binding to CRF receptors can be conducted using radioligands [125I]Tyr0-CRF or [125I]Tyr0-sauvagine with Kd values of 0.1-0.4 nM. CRF1 and CRF2 receptors are non-selectively antagonized by α-helical CRF, D-Phe-CRF-(12-41) and astressin. CRF1 receptors are selectively antagonized by small molecules NBI27914, R121919, antalarmin, CP 154,526, CP 376,395. CRF2 receptors are selectively antagonized by antisauvagine and astressin 2B. Although selective small molecule CRF1 receptor antagonists were not effective in treating major depressive disorder, posttraumatic stress disorder, or alcohol use disorder in clinical trials, recent phase 2 studies have found that CRF1 receptor antagonists effectively reduce adrenocortical androgens and precursors in congentical adrenal hyperplasia [61]

Type VIII RTKs: ROR family in GtoPdb v.2025.3

ROR1 and ROR2 are involved in regulating Wnt-5a signalling. There is evidence that ROR1 and ROR2 can form heteromeric complexes. Due to their role in cancer, therapies targeting RORs are under investigation. Thus, ROR1 and ROR2 appear to be activated by Wnt-5a binding to a Frizzled receptor thereby forming a cell-surface multiprotein complex [3]

S33: Prolyl aminopeptidase in GtoPdb v.2025.3

Peptidase family S33 contains mainly exopeptidases that act at the N-terminus of peptides

Glucagon receptor family in GtoPdb v.2025.3

The glucagon family of receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on the Glucagon receptor family [170]) are activated by the endogenous peptide (27-44 aa) hormones glucagon, glucagon-like peptide 1, glucagon-like peptide 2, glucose-dependent insulinotropic polypeptide (also known as gastric inhibitory polypeptide), GHRH and secretin. One common precursor (GCG) generates glucagon, glucagon-like peptide 1 and glucagon-like peptide 2 peptides [126]. For a recent review on the current understanding of the structures of GLP-1 and GLP-1R, the molecular basis of their interaction, and the associated signaling events see de Graaf et al., 2016 [94]

3C. 3-Ketosteroid receptors in GtoPdb v.2025.3

Steroid hormone receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Nuclear Hormone Receptors [75, 221, 3]) are nuclear hormone receptors of the NR3 class, with endogenous agonists that may be divided into 3-hydroxysteroids (estrone and 17β-estradiol) and 3-ketosteroids (dihydrotestosterone [DHT], aldosterone, cortisol, corticosterone, progesterone and testosterone). For rodent GR and MR, the physiological ligand is corticosterone rather than cortisol. Clinically-used drugs interacting with the 3-ketosteroid receptors include testosterone (AR), methylprednisolone (GR), eplerenone (MR), and medroxyprogesterone (PR)