COMMD1 regulates the delta epithelial sodium channel (δENaC) through trafficking and ubiquitination

Lead exposure disturbs ATP7B-mediated copper export from brain barrier cells by inhibiting XIAP-regulated COMMD1 protein degradation

The brain barrier is an important structure for metal ion homeostasis. According to studies, lead (Pb) exposure disrupts the transportation of copper (Cu) through the brain barrier, which may cause impairment of the nervous system; however, the specific mechanism is unknown. The previous studies suggested the X-linked inhibitor of apoptosis (XIAP) is a sensor for cellular Cu level which mediate the degradation of the MURR1 domain-containing 1 (COMMD1) protein. XIAP/COMMD1 axis was thought to be an important regulator in Cu metabolism maintenance. In this study, the role of XIAP-regulated COMMD1 protein degradation in Pb-induced Cu disorders in brain barrier cells was investigated. Pb exposure significantly increased Cu levels in both cell types, according to atomic absorption technology testing. Western blotting and reverse transcription PCR (RT-PCR) showed that COMMD1 protein levels were significantly increased, whereas XIAP, ATP7A, and ATP7B protein levels were significantly decreased. However, there were no significant effects at the messenger RNA (mRNA) level (XIAP, ATP7A, and ATP7B). Pb-induced Cu accumulation and ATP7B expression were reduced when COMMD1 was knocked down by transient small interfering RNA (siRNA) transfection. In addition, transient plasmid transfection of XIAP before Pb exposure reduced Pb-induced Cu accumulation, increased COMMD1 protein levels, and decreased ATP7B levels. In conclusion, Pb exposure can reduce XIAP protein expression, increase COMMD1 protein levels, and specifically decrease ATP7B protein levels, resulting in Cu accumulation in brain barrier cells.

COMMD proteins function and their regulating roles in tumors

The COMMD proteins are a highly conserved protein family with ten members that play a crucial role in a variety of biological activities, including copper metabolism, endosomal sorting, ion transport, and other processes. Recent research have demonstrated that the COMMD proteins are closely associated with a wide range of disorders, such as hepatitis, myocardial ischemia, cerebral ischemia, HIV infection, and cancer. Among these, the role of COMMD proteins in tumors has been thoroughly explored; they promote or inhibit cancers such as lung cancer, liver cancer, gastric cancer, and prostate cancer. COMMD proteins can influence tumor proliferation, invasion, metastasis, and tumor angiogenesis, which are strongly related to the prognosis of tumors and are possible therapeutic targets for treating tumors. In terms of molecular mechanism, COMMD proteins in tumor cells regulate the oncogenes of NF-κB, HIF, c-MYC, and others, and are related to signaling pathways including apoptosis, autophagy, and ferroptosis. For the clinical diagnosis and therapy of malignancies, additional research into the involvement of COMMD proteins in cancer is beneficial.

Commander Complex-A Multifaceted Operator in Intracellular Signaling and Cargo

Commander complex is a 16-protein complex that plays multiple roles in various intracellular events in endosomal cargo and in the regulation of cell homeostasis, cell cycle and immune response. It consists of COMMD1-10, CCDC22, CCDC93, DENND10, VPS26C, VPS29, and VPS35L. These proteins are expressed ubiquitously in the human body, and they have been linked to diseases including Wilson's disease, atherosclerosis, and several types of cancer. In this review we describe the function of the commander complex in endosomal cargo and summarize the individual known roles of COMMD proteins in cell signaling and cancer. It becomes evident that commander complex might be a much more important player in intracellular regulation than we currently understand, and more systematic research on the role of commander complex is required.

Transcriptional analysis of the expression, prognostic value and immune infiltration activities of the COMMD protein family in hepatocellular carcinoma

BACKGROUND

The copper metabolism MURR1 domain (COMMD) protein family involved in tumor development and progression in several types of human cancer, but little is known about the function of COMMD proteins in hepatocellular carcinoma (HCC).

METHODS

The ONCOMINE and the UALCAN databases were used to evaluate the expression of COMMD1-10 in HCC and the association of this family with individual cancer stage and tumor grade. Kaplan-Meier (K-M) Plotter and Cox analysis hint the prognostic value of COMMDs. A network comprising 50 most similar genes and COMMD1-10 was constructed with the STRING database. Gene set enrichment analysis (GSEA) was performed using LinkedOmics database. The correlations between COMMD expression and the presence of immune infiltrating cells were also analyzed by the tumor immune estimation resource (TIMER) database. GSE14520 dataset and 80 HCC patients were used to validated the expression and survival value of COMMD3. Human HCC cell lines were also used for validating the function of COMMD3.

RESULTS

The expression of all COMMD family members showed higher expression in HCC tissues than that in normal tissues, and is associated with clinical cancer stage and pathological tumor grade. In HCC patients, the transcriptional levels of COMMD1/4 are positively correlated with overall survival (OS), while those of COMMD2/3/7/8/9 are negatively correlated with OS. Multivariate analysis indicated that a high level of COMMD3 mRNA is an independent prognostic factor for shorter OS in HCC patients. However, the subset of patients with grade 3 HCC, K-M survival curves revealed that high COMMD3/5/7/8/9 expression and low COMMD4/10 expression were associated with shorter OS. In addition, the expression of COMMD2/3/10 was associated with tumor-induced immune response activation and immune infiltration in HCC. The expression of COMMD3 from GSE14520 dataset and 80 patients are both higher in tumor than that in normal tissue, and a higher level of COMMD3 mRNA is associated with shorter OS. Knockdown of COMMD3 inhibits human HCC cell lines proliferation in vitro.

CONCLUSIONS

Our study indicates that COMMD3 is an independent prognostic biomarker for the survival of HCC patients. COMMD3 supports the proliferation of HCC cells and contributes to the poor OS in HCC patients.

COMMD1, a multi-potent intracellular protein involved in copper homeostasis, protein trafficking, inflammation, and cancer

Copper is a trace element indispensable for life, but at the same time it is implicated in reactive oxygen species formation. Several inherited copper storage diseases are described of which Wilson disease (copper overload, mutations in ATP7B gene) and Menkes disease (copper deficiency, mutations in ATP7A gene) are the most prominent ones. After the discovery in 2002 of a novel gene product (i.e. COMMD1) involved in hepatic copper handling in Bedlington terriers, studies on the mechanism of action of COMMD1 revealed numerous non-copper related functions. Effects on hepatic copper handling are likely mediated via interactions with ATP7B. In addition, COMMD1 has many more interacting partners which guide their routing to either the plasma membrane or, often in an ubiquitination-dependent fashion, trigger their proteolysis via the S26 proteasome. By stimulating NF-κB ubiquitination, COMMD1 dampens an inflammatory reaction. Finally, targeting COMMD1 function can be a novel approach in the treatment of tumors.

Identification of the COMM-domain containing protein 1 as specific binding partner for the guanine-rich RNA sequence binding factor 1

BACKGROUND

The guanine-rich RNA sequence binding factor 1 (GRSF1) is an RNA-binding protein of the hnRNP H/F family, which has been implicated in erythropoiesis, regulation of the redox homeostasis, embryonic brain development, mitochondrial function and cellular senescence. The molecular basis for GRSF1-RNA interaction has extensively been studied in the past but for the time being GRSF1 binding proteins have not been identified.

METHODS

To search for GRSF1 binding proteins we first employed the yeast two-hybrid system and screened a cDNA library of human fetal brain for potential GRSF1 binding proteins. Subsequently, we explored the protein-protein-interaction of the recombiant proteins, carried out immunoprecipitation experiments to confirm the interaction of the native proteins in living cells and performed truncation studies to identify the protein-binding motif of GRSF1.

RESULTS

Using the yeast two-hybrid system we identified the COMM-domain containing protein 1 (COMMD1) as specific GRSF1 binding protein and in vitro truncation studies suggested that COMMD1 interacts with the alanine-rich domain of GRSF1. Co-immunoprecipitation strategies indicated that COMMD1-GRSF1 interaction was RNA independent and also occurred in living cells expressing the two native proteins.

CONCLUSION

In mammalian cells the COMM-domain containing protein 1 (COMMD1) specifically interacts with the Ala-rich domain of GRSF1 in an RNA-independent manner.

GENERAL SIGNIFICANCE

This is the first report describing a specific GRSF1 binding protein.

COMMD8 changes expression during initial phase of wasp venom immunotherapy

BACKGROUND

Hymenoptera venom allergy (HVA) is of great concern because of the possibility of anaphylaxis, which may be fatal. Venom immunotherapy (VIT) is the only disease-modifying treatment in HVA and, although efficient, its mechanism remains partially unknown. Gene expression analysis may be helpful for establishing a proper model of tolerance induction during the build-up phase of VIT. The present study aimed to analyze how the start of VIT changes the expression of 15 selected genes.

METHODS

Forty-five patients starting VIT with a wasp venom allergy were enrolled. The diagnosis was established based on anaphylaxis history (third or fourth grade on the Mueller scale) and positive soluble immunoglobulin E and/or skin tests. Two blood collections were performed in the patient group: before and after 3 months of VIT. One sample was taken in the control group. Gene expression analysis was performed using a reverse transcriptase-polymerase chain reaction with microfluidic cards and normalized to the 18S housekeeping gene.

RESULTS

Commd8 was the only gene that changed expression significantly after the start of VIT (p = 0.012). Its expression decreased towards the levels observed in the healthy controls. Twelve out of 15 genes (commd8, cldn1, cngb3, fads1, hes6, hla-drb5, htr3b, prlr, slc16a4, snx33, socs3 and twist2) revealed a significantly different expression compared to the healthy controls.

CONCLUSIONS

The present study shows that commd8 changes significantly its expression during initial phase of VIT. This gene might be a candidate for VIT biomarker in future studies.

Systems-wide Studies Uncover Commander, a Multiprotein Complex Essential to Human Development

Recent mass spectrometry maps of the human interactome independently support the existence of a large multiprotein complex, dubbed "Commander." Broadly conserved across animals and ubiquitously expressed in nearly every human cell type examined thus far, Commander likely plays a fundamental cellular function, akin to other ubiquitous machines involved in expression, proteostasis, and trafficking. Experiments on individual subunits support roles in endosomal protein sorting, including the trafficking of Notch proteins, copper transporters, and lipoprotein receptors. Commander is critical for vertebrate embryogenesis, and defects in the complex and its interaction partners disrupt craniofacial, brain, and heart development. Here, we review the synergy between large-scale proteomic efforts and focused studies in the discovery of Commander, describe its composition, structure, and function, and discuss how it illustrates the power of systems biology. Based on 3D modeling and biochemical data, we draw strong parallels between Commander and the retromer cargo-recognition complex, laying a foundation for future research into Commander's role in human developmental disorders.

Epithelial Na+ Channel: Reciprocal Control by COMMD10 and Nedd4-2

Optimal function of the epithelial sodium channel (ENaC) in the distal nephron is key to the kidney’s long-term control of salt homeostasis and blood pressure. Multiple pathways alter ENaC cell surface populations, including correct processing and trafficking in the secretory pathway to the cell surface, and retrieval from the cell surface through ubiquitination by the ubiquitin ligase Nedd4-2, clathrin-mediated endocytosis, and sorting in the endosomal system. Members of the Copper Metabolism Murr1 Domain containing (COMMD) family of 10 proteins are known to interact with ENaC. COMMD1, 3 and 9 have been shown to down-regulate ENaC, most likely through Nedd4-2, however, the other COMMD family members remain uncharacterized. To investigate the effects of the COMMD10 protein on ENaC trafficking and function, the interaction of ENaC and COMMD10 was confirmed. Stable COMMD10 knockdown in Fischer rat thyroid epithelia decreased ENaC current and this decreased current was associated with increased Nedd4-2 protein, a known negative regulator of ENaC. However, inhibition of Nedd4-2’s ubiquitination of ENaC was only able to partially rescue the observed reduction in current. Stable COMMD10 knockdown results in defects both in endocytosis and recycling of transferrin suggesting COMMD10 likely interacts with multiple pathways to regulate ENaC and therefore could be involved in the long-term control of blood pressure.

COMMD1/Murr1 reinforces HIV-1 latent infection through IκB-α stabilization

The transcription factor NF-κB is important for HIV-1 transcription initiation in primary HIV-1 infection and reactivation in latently HIV-1-infected cells. However, comparative analysis of the regulation and function of NF-κB in latently HIV-1-infected cells has not been done. Here we show that the expression of IκB-α, an endogenous inhibitor of NF-κB, is enhanced by latent HIV-1 infection via induction of the host-derived factor COMMD1/Murr1 in myeloid cells but not in lymphoid cells by using four sets of latently HIV-1-infected cells and the respective parental cells. IκB-α protein was stabilized by COMMD1, which attenuated NF-κB signaling during Toll-like receptor ligand and tumor necrosis factor alpha treatment and enhanced HIV-1 latency in latently HIV-1-infected cells. Activation of the phosphoinositol 3-kinase (PI3K)-JAK pathway is involved in COMMD1 induction in latently HIV-1-infected cells. Our findings indicate that COMMD1 induction is the NF-κB inhibition mechanism in latently HIV-1-infected cells that contributes to innate immune deficiency and reinforces HIV-1 latency. Thus, COMMD1 might be a double-edged sword that is beneficial in primary infection but not beneficial in latent infection when HIV-1 eradication is considered.

IMPORTANCE

HIV-1 latency is a major barrier to viral eradication in the era of combination antiretroviral therapy. In this study, we found that COMMD1/Murr1, previously identified as an HIV-1 restriction factor, inhibits the proteasomal degradation of IκB-α by increasing the interaction with IκB-α in latently HIV-1-infected myeloid cells. IκB-α protein was stabilized by COMMD1, which attenuated NF-κB signaling during the innate immune response and enhanced HIV-1 latency in latently HIV-1-infected cells. Activation of the PI3K-JAK pathway is involved in COMMD1 induction in latently HIV-1-infected cells. Thus, the host-derived factor COMMD1 is beneficial in suppressing primary infection but enhances latent infection, indicating that it may be a double-edged sword in HIV-1 eradication.

A cell-type-specific role for murine Commd1 in liver inflammation

The transcription factor NF-κB plays a critical role in the inflammatory response and it has been implicated in various diseases, including non-alcoholic fatty liver disease (NAFLD). Although transient NF-κB activation may protect tissues from stress, a prolonged NF-κB activation can have a detrimental effect on tissue homeostasis and therefore accurate termination is crucial. Copper Metabolism MURR1 Domain-containing 1 (COMMD1), a protein with functions in multiple pathways, has been shown to suppress NF-κB activity. However, its action in controlling liver inflammation has not yet been investigated. To determine the cell-type-specific contribution of Commd1 to liver inflammation, we used hepatocyte and myeloid-specific Commd1-deficient mice. We also used a mouse model of NAFLD to study low-grade chronic liver inflammation: we fed the mice a high fat, high cholesterol (HFC) diet, which results in hepatic lipid accumulation accompanied by liver inflammation. Depletion of hepatocyte Commd1 resulted in elevated levels of the NF-κB transactivation subunit p65 (RelA) but, surprisingly, the level of liver inflammation was not aggravated. In contrast, deficiency of myeloid Commd1 exacerbated diet-induced liver inflammation. Unexpectedly we observed that hepatic and myeloid Commd1 deficiency in the mice both augmented hepatic lipid accumulation. The elevated levels of proinflammatory cytokines in myeloid Commd1-deficient mice might be responsible for the increased level of steatosis. This increase was not seen in hepatocyte Commd1-deficient mice, in which increased lipid accumulation appeared to be independent of inflammation. Our mouse models demonstrate a cell-type-specific role for Commd1 in suppressing liver inflammation and in the progression of NAFLD.

Copper metabolism domain-containing 1 represses genes that promote inflammation and protects mice from colitis and colitis-associated cancer

BACKGROUND & AIMS

Activation of the transcription factor nuclear factor-κB (NF-κB) has been associated with the development of inflammatory bowel disease (IBD). Copper metabolism MURR1 domain containing 1 (COMMD1), a regulator of various transport pathways, has been shown to limit NF-κB activation. We investigated the roles of COMMD1 in the pathogenesis of colitis in mice and IBD in human beings.

METHODS

We created mice with a specific disruption of Commd1 in myeloid cells (Mye-knockout [K/O] mice); we analyzed immune cell populations and functions and expression of genes regulated by NF-κB. Sepsis was induced in Mye-K/O and wild-type mice by cecal ligation and puncture or intraperitoneal injection of lipopolysaccharide (LPS), colitis was induced by administration of dextran sodium sulfate, and colitis-associated cancer was induced by administration of dextran sodium sulfate and azoxymethane. We measured levels of COMMD1 messenger RNA in colon biopsy specimens from 29 patients with IBD and 16 patients without (controls), and validated findings in an independent cohort (17 patients with IBD and 22 controls). We searched for polymorphisms in or near COMMD1 that were associated with IBD using data from the International IBD Genetics Consortium and performed quantitative trait locus analysis.

RESULTS

In comparing gene expression patterns between myeloid cells from Mye-K/O and wild-type mice, we found that COMMD1 represses expression of genes induced by LPS. Mye-K/O mice had more intense inflammatory responses to LPS and developed more severe sepsis and colitis, with greater mortality. More Mye-K/O mice with colitis developed colon dysplasia and tumors than wild-type mice. We observed a reduced expression of COMMD1 in colon biopsy specimens and circulating leukocytes from patients with IBD. We associated single-nucleotide variants near COMMD1 with reduced expression of the gene and linked them with increased risk for ulcerative colitis.

CONCLUSIONS

Expression of COMMD1 by myeloid cells has anti-inflammatory effects. Reduced expression or function of COMMD1 could be involved in the pathogenesis of IBD.

New canine models of copper toxicosis: diagnosis, treatment, and genetics

The One Health principle recognizes that human health, animal health, and environmental health are inextricably linked. An excellent example is the study of naturally occurring copper toxicosis in dogs to help understand human disorders of copper metabolism. Besides the Bedlington terrier, where copper toxicosis is caused by a mutation in the COMMD1 gene, more complex hereditary forms of copper-associated hepatitis were recognized recently in other dog breeds. The Labrador retriever is one such breed, where an interplay between genetic susceptibility and exposure to copper lead to clinical copper toxicosis. Purebred dog populations are ideal for gene mapping studies, and because genes involved in copper metabolism are highly conserved across species, newly identified gene mutations in the dog may help unravel the genetic complexity of different human forms of copper toxicosis. Furthermore, increasing knowledge with respect to diagnosis and treatment strategies will benefit both species.

Blood pressure and amiloride-sensitive sodium channels in vascular and renal cells

Sodium transport in the distal nephron is mediated by epithelial sodium channel activity. Proteolytic processing of external domains and inhibition with increased sodium concentrations are important regulatory features of epithelial sodium channel complexes expressed in the distal nephron. By contrast, sodium channels expressed in the vascular system are activated by increased external sodium concentrations, which results in changes in the mechanical properties and function of endothelial cells. Mechanosensitivity and shear stress affect both epithelial and vascular sodium channel activity. Guyton's hypothesis stated that blood pressure control is critically dependent on vascular tone and fluid handling by the kidney. The synergistic effects, and complementary regulation, of the epithelial and vascular systems are consistent with the Guytonian model of volume and blood pressure regulation, and probably reflect sequential evolution of the two systems. The integration of vascular tone, renal perfusion and regulation of renal sodium reabsorption is the central underpinning of the Guytonian model. In this Review, we focus on the expression and regulation of sodium channels, and we outline the emerging evidence that describes the central role of amiloride-sensitive sodium channels in the efferent (vascular) and afferent (epithelial) arms of this homeostatic system.

Regulation of the delta and alpha epithelial sodium channel (ENaC) by ubiquitination and Nedd8

The δ epithelial sodium channel (δENaC) is a proton-activated, sodium-selective, amiloride-sensitive ion channel in the ENaC/degenerin family of ion channels involved in blood pressure regulation and mechanosensation. Other ENaC family members are subject to ubiquitin modification leading to internalization from the cell surface, and degradation of the channel. Here, we show that δENaC is also modified by ubiquitin on three intracellular lysine residues. Absence of these lysines abolished ubiquitin modification of δENaC and increased cell surface levels of δENaC. Although the HECT-domain ubiquitin ligase Nedd4-2 reduced amiloride-sensitive current generated by δβγENaC-containing channels, δENaC does not contain a binding site for Nedd4-2; therefore, this effect is probably mediated by the βγENaC subunits. Nedd8, a ubiquitin-like protein that regulates RING-domain E3 ubiquitin ligases, promoted δENaC ubiquitination, decreased both the intracellular and cell surface δENaC populations, and decreased δβγENaC amiloride-sensitive short circuit current (Isc -amiloride) in a mammalian epithelium. Nedd8 also promoted α- and γENaC ubiquitination, decreased the cell surface pools, and decreased αβγENaC Isc -amiloride. Conversely, XIAP, a single subunit RING E3 ligase, decreased ubiquitinated δENaC, increased the δENaC cell surface pool and increased δβγENaC Isc -amiloride. Therefore δ- and α - βγENaC channel function may be influenced by RING-domain E3 ubiquitin ligases.

Tuning NF-κB activity: a touch of COMMD proteins

NF-κB is an important regulator of immunity and inflammation, and its activation pathway has been studied extensively. The mechanisms that downregulate the activity of NF-κB have also received a lot of attention, particularly since its activity needs to be terminated to prevent chronic inflammation and subsequent tissue damage. The COMMD family has been identified as a new group of proteins involved in NF-κB termination. All ten COMMD members share the structurally conserved carboxy-terminal motif, the COMM domain, and are ubiquitously expressed. They seem to play distinct and non-redundant roles in various physiological processes, including NF-κB signaling. In this review, we describe the mechanisms and proteins involved in the termination of canonical NF-κB signaling, with a specific focus on the role of the COMMD family in the down-modulation of NF-κB.

CCDC22 deficiency in humans blunts activation of proinflammatory NF-κB signaling

NF-κB is a master regulator of inflammation and has been implicated in the pathogenesis of immune disorders and cancer. Its regulation involves a variety of steps, including the controlled degradation of inhibitory IκB proteins. In addition, the inactivation of DNA-bound NF-κB is essential for its regulation. This step requires a factor known as copper metabolism Murr1 domain-containing 1 (COMMD1), the prototype member of a conserved gene family. While COMMD proteins have been linked to the ubiquitination pathway, little else is known about other family members. Here we demonstrate that all COMMD proteins bind to CCDC22, a factor recently implicated in X-linked intellectual disability (XLID). We showed that an XLID-associated CCDC22 mutation decreased CCDC22 protein expression and impaired its binding to COMMD proteins. Moreover, some affected individuals displayed ectodermal dysplasia, a congenital condition that can result from developmental NF-κB blockade. Indeed, patient-derived cells demonstrated impaired NF-κB activation due to decreased IκB ubiquitination and degradation. In addition, we found that COMMD8 acted in conjunction with CCDC22 to direct the degradation of IκB proteins. Taken together, our results indicate that CCDC22 participates in NF-κB activation and that its deficiency leads to decreased IκB turnover in humans, highlighting an important regulatory component of this pathway.

The Antitumor Peptide CIGB-552 Increases COMMD1 and Inhibits Growth of Human Lung Cancer Cells

We have demonstrated that the peptide L-2 designed from an alanine scanning of the Limulus-derived LALF32-51 region is a potential candidate for the anticancer therapy and its cell-penetrating capacity is an associated useful property. By the modification in the primary structure of L-2, a second-generation peptide (CIGB-552) was developed. However, the molecular mechanism underlying its cytotoxic activity remains partially unknown. In this study, it was shown that CIGB-552 increases the levels of COMMD1, a protein involved in copper homeostasis, sodium transport, and the NF-κB signaling pathway. We found that CIGB-552 induces ubiquitination of RelA and inhibits the antiapoptotic activity regulated by NF-κB, whereas the knockdown of COMMD1 blocks this effect. We also found that CIGB-552 decreases the antioxidant capacity and induces the peroxidation of proteins and lipids in the tumor cells. Altogether, this study provides new insights into the mechanism of action of the peptide CIGB-552, which could be relevant in the design of future anticancer therapies.

δ ENaC: a novel divergent amiloride-inhibitable sodium channel

The fourth subunit of the epithelial sodium channel, termed delta subunit (δ ENaC), was cloned in human and monkey. Increasing evidence shows that this unique subunit and its splice variants exhibit biophysical and pharmacological properties that are divergent from those of α ENaC channels. The widespread distribution of epithelial sodium channels in both epithelial and nonepithelial tissues implies a range of physiological functions. The altered expression of SCNN1D is associated with numerous pathological conditions. Genetic studies link SCNN1D deficiency with rare genetic diseases with developmental and functional disorders in the brain, heart, and respiratory systems. Here, we review the progress of research on δ ENaC in genomics, biophysics, proteomics, physiology, pharmacology, and clinical medicine.

The epithelial sodium channel δ-subunit: new notes for an old song

Amiloride-sensitive epithelial Na(+) channels (ENaCs) can be formed by different combinations of four homologous subunits, named α, β, γ, and δ. In addition to providing an apical entry pathway for transepithelial Na(+) reabsorption in tight epithelia such as the kidney distal tubule and collecting duct, ENaCs are also expressed in nonepithelial cells, where they may play different functional roles. The δ-subunit of ENaC was originally identified in humans and is able to form amiloride-sensitive Na(+) channels alone or in combination with β and γ, generally resembling the canonical kidney ENaC formed by α, β, and γ. However, δ differs from α in its tissue distribution and channel properties. Despite the low sequence conservation between α and δ (37% identity), their similar functional characteristics provide an excellent model for exploring structural correlates of specific ENaC biophysical and pharmacological properties. Moreover, the study of cellular mechanisms modulating the activity of different ENaC subunit combinations provides an opportunity to gain insight into the regulation of the channel. In this review, we examine the evolution of ENaC genes, channel subunit composition, the distinct functional and pharmacological features that δ confers to ENaC, and how this can be exploited to better understand this ion channel. Finally, we briefly consider possible functional roles of the ENaC δ-subunit.

Characterization of a novel splice variant of δ ENaC subunit in human lungs

Salt absorption via apical epithelial sodium channels (ENaC) is a critical rate-limiting process in maintaining airway and lung lining fluid at the physiological level. δ ENaC (termed δ1 in this article) has been detected in human lung epithelial cells in addition to α, β, and γ subunits (Ji HL, Su XF, Kedar S, Li J, Barbry P, Smith PR, Matalon S, Benos DJ. J Biol Chem 281: 8233-8241, 2006; Nie HG, Chen L, Han DY, Li J, Song WF, Wei SP, Fang XH, Gu X, Matalon S, Ji HL, J Physiol 587: 2663-2676, 2009) and may contribute to the differences in the biophysical properties of amiloride-inhibitable cation channels in pulmonary epithelial cells. Here we cloned a splicing variant of the δ1 ENaC, namely, δ2 ENaC in human bronchoalveolar epithelial cells (16HBEo). δ2 ENaC possesses 66 extra amino acids attached to the distal amino terminal tail of the δ1 ENaC. δ2 ENaC was expressed in both alveolar type I and II cells of human lungs as revealed by in situ hybridization and real-time RT-PCR. To characterize the biophysical and pharmacological features of the splicing variant, we injected Xenopus oocytes with human ENaC cRNAs and measured whole cell and single channel currents of δ1βγ, δ2βγ, and αβγ channels. Oocytes injected with δ2βγ cRNAs exhibited whole cell currents significantly greater than those expressing δ1βγ and αβγ channels. Single channel activity, unitary conductance, and open probability of δ2βγ channels were significantly greater compared with δ1βγ and αβγ channels. In addition, δ2βγ and δ1βγ channels displayed significant differences in apparent Na(+) affinity, dissociation constant for amiloride (K(i)(amil)), the EC(50) for capsazepine activation, and gating kinetics by protons. Channels comprising of this novel splice variant may contribute to the diversities of native epithelial Na(+) channels.

Canine models of copper toxicosis for understanding mammalian copper metabolism

Hereditary forms of copper toxicosis exist in man and dogs. In man, Wilson's disease is the best studied disorder of copper overload, resulting from mutations in the gene coding for the copper transporter ATP7B. Forms of copper toxicosis for which no causal gene is known yet are recognized as well, often in young children. Although advances have been made in unraveling the genetic background of disorders of copper metabolism in man, many questions regarding disease mechanisms and copper homeostasis remain unanswered. Genetic studies in the Bedlington terrier, a dog breed affected with copper toxicosis, identified COMMD1, a gene that was previously unknown to be involved in copper metabolism. Besides the Bedlington terrier, a number of other dog breeds suffer from hereditary copper toxicosis and show similar phenotypes to humans with copper storage disorders. Unlike the heterogeneity of most human populations, the genetic structure within a purebred dog population is homogeneous, which is advantageous for unraveling the molecular genetics of complex diseases. This article reviews the work that has been done on the Bedlington terrier, summarizes what was learned from studies into COMMD1 function, describes hereditary copper toxicosis phenotypes in other dog breeds, and discusses the opportunities for genome-wide association studies on copper toxicosis in the dog to contribute to the understanding of mammalian copper metabolism and copper metabolism disorders in man.

Differential N termini in epithelial Na+ channel δ-subunit isoforms modulate channel trafficking to the membrane

The epithelial Na(+) channel (ENaC) is a heteromultimeric ion channel that plays a key role in Na(+) reabsorption across tight epithelia. The canonical ENaC is formed by three analogous subunits, α, β, and γ. A fourth ENaC subunit, named δ, is expressed in the nervous system of primates, where its role is unknown. The human δ-ENaC gene generates at least two splice isoforms, δ(1) and δ(2) , differing in the N-terminal sequence. Neurons in diverse areas of the human and monkey brain differentially express either δ(1) or δ(2) , with few cells coexpressing both isoforms, which suggests that they may play specific physiological roles. Here we show that heterologous expression of δ(1) in Xenopus oocytes and HEK293 cells produces higher current levels than δ(2) . Patch-clamp experiments showed no differences in single channel current magnitude and open probability between isoforms. Steady-state plasma membrane abundance accounts for the dissimilarity in macroscopic current levels. Differential trafficking between isoforms is independent of β- and γ-subunits, PY-motif-mediated endocytosis, or the presence of additional lysine residues in δ(2)-N terminus. Analysis of δ(2)-N terminus identified two sequences that independently reduce channel abundance in the plasma membrane. The δ(1) higher abundance is consistent with an increased insertion rate into the membrane, since endocytosis rates of both isoforms are indistinguishable. Finally, we conclude that δ-ENaC undergoes dynamin-independent endocytosis as opposed to αβγ-channels.