Icilin Activates the δ-Subunit of the Human Epithelial Na+ Channel

Joint CB1 and NGF Receptor Activation Suppresses TRPM8 Activation in Etoposide-Resistant Retinoblastoma Cells

In childhood, retinoblastoma (RB) is the most common primary tumor in the eye. Long term therapeutic management with etoposide of this life-threatening condition may have diminishing effectiveness since RB cells can develop cytostatic resistance to this drug. To determine whether changes in receptor-mediated control of Ca2+ signaling are associated with resistance development, fluorescence calcium imaging, semi-quantitative RT-qPCR analyses, and trypan blue dye exclusion staining patterns are compared in WERI-ETOR (etoposide-insensitive) and WERI-Rb1 (etoposide-sensitive) cells. The cannabinoid receptor agonist 1 (CNR1) WIN55,212-2 (40 µM), or the transient receptor potential melastatin 8 (TRPM8) agonist icilin (40 µM) elicit similar large Ca2+ transients in both cell line types. On the other hand, NGF (100 ng/mL) induces larger rises in WERI-ETOR cells than in WERI-Rb1 cells, and its lethality is larger in WERI-Rb1 cells than in WERI-ETOR cells. NGF and WIN55,212-2 induced additive Ca2+ transients in both cell types. However, following pretreatment with both NGF and WIN55,212-2, TRPM8 gene expression declines and icilin-induced Ca2+ transients are completely blocked only in WERI-ETOR cells. Furthermore, CNR1 gene expression levels are larger in WERI-ETOR cells than those in WERI-Rb1 cells. Therefore, the development of etoposide insensitivity may be associated with rises in CNR1 gene expression, which in turn suppress TRPM8 gene expression through crosstalk.

Capsazepine activates amiloride-insensitive ENaC-like channels in human leukemia cells

Sodium influx carried out by ion channels is one of the main regulators of water-salt and volume balance in cells of blood origin. Previously, we described amiloride-insensitive ENaC-like channels in human myeloid leukemia K562 cells; the intracellular regulatory mechanisms of the channels are associated with actin cytoskeleton dynamics. Recently, an extracellular mechanism of ENaC-like channels activation in K562 cells by the action of serine protease trypsin has been revealed. The other extracellular pathways that modulate ENaC (epithelial Na+ channel) activity and sodium permeability in transformed blood cells are not yet fully investigated. Here, we study the action of capsazepine (CPZ), as δ-ENaC activator, on single channel activity in K562 cells in whole-cell patch clamp experiments. Addition of CPZ (2 μM) to the extracellular solution caused an activation of sodium channels with typical features; unitary conductance was 15.1 ± 0.8 pS. Amiloride derivative benzamil (50 μM) did not inhibit their activity. Unitary currents and conductance of CPZ-activated channels were higher in Na+-containing extracellular solution than in Li+, that is one of the main fingerprints of δ-ENaC. The results of RT-PCR analysis and immunofluorescence staining also confirmed the expression of δ-hENaC (as well as α-, β-, γ-ENaC) at the mRNA and protein level. These findings allow us to speculate that CPZ activates amiloride-insensitive ENaC-like channels that contain δ-ENaC in К562 cells. Our data reveal a novel extracellular mechanism for ENaC-like activation in human leukemia cells.

The subunit of epithelial sodium channel in humans-a potential player in vascular physiology

Vascular epithelial sodium channels (ENaCs) made up of canonical α, β, and γ subunits have attracted more attention recently owing to their physiological role in vascular health and disease. A fourth subunit, δ-ENaC, is expressed in various mammalian species, except mice and rats, which are common animal models for cardiovascular research. Accordingly, δ-ENaC is the least understood subunit. However, the recent discovery of δ subunit in human vascular cells indicates that this subunit may play a significant role in normal/pathological vascular physiology in humans. Channels containing the δ subunit have different biophysical and pharmacological properties compared with channels containing the α subunit, with the potential to alter the vascular function of ENaC in health and disease. Hence, it is important to investigate the expression and function of δ-ENaC in the vasculature to identify whether δ-ENaC is a potential new drug target for the treatment of cardiovascular disease. In this review, we will focus on the existing knowledge of δ-ENaC and implications for vascular physiology and pathophysiology in humans.

Activation of the Human Epithelial Sodium Channel (ENaC) by Bile Acids Involves the Degenerin Site

The epithelial sodium channel (ENaC) is a member of the ENaC/degenerin ion channel family, which also includes the bile acid-sensitive ion channel (BASIC). So far little is known about the effects of bile acids on ENaC function. ENaC is probably a heterotrimer consisting of three well characterized subunits (αβγ). In humans, but not in mice and rats, an additional δ-subunit exists. The aim of this study was to investigate the effects of chenodeoxycholic, cholic, and deoxycholic acid in unconjugated (CDCA, CA, and DCA) and tauro-conjugated (t-CDCA, t-CA, t-DCA) form on human ENaC in its αβγ- and δβγ-configuration. We demonstrated that tauro-conjugated bile acids significantly stimulate ENaC in the αβγ- and in the δβγ-configuration. In contrast, non-conjugated bile acids have a robust stimulatory effect only on δβγENaC. Bile acids stimulate ENaC-mediated currents by increasing the open probability of active channels without recruiting additional near-silent channels known to be activated by proteases. Stimulation of ENaC activity by bile acids is accompanied by a significant reduction of the single-channel current amplitude, indicating an interaction of bile acids with a region close to the channel pore. Analysis of the known ASIC1 (acid-sensing ion channel) crystal structure suggested that bile acids may bind to the pore region at the degenerin site of ENaC. Substitution of a single amino acid residue within the degenerin region of βENaC (N521C or N521A) significantly reduced the stimulatory effect of bile acids on ENaC, suggesting that this site is critical for the functional interaction of bile acids with the channel.

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.

Amiloride-sensitive channels are a major contributor to mechanotransduction in mammalian muscle spindles

We investigated whether channels of the epithelial sodium/amiloride-sensitive degenerin (ENaC/DEG) family are a major contributor to mechanosensory transduction in primary mechanosensory afferents, using adult rat muscle spindles as a model system. Stretch-evoked afferent discharge was reduced in a dose-dependent manner by amiloride and three analogues - benzamil, 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and hexamethyleneamiloride (HMA), reaching > or = 85% inhibition at 1 mm. Moreover, firing was slightly but significantly increased by ENaC delta subunit agonists (icilin and capsazepine). HMA's profile of effects was distinct from that of the other drugs. Amiloride, benzamil and EIPA significantly decreased firing (P < 0.01 each) at 1 microm, while 10 microm HMA was required for highly significant inhibition (P < 0.0001). Conversely, amiloride, benzamil and EIPA rarely blocked firing entirely at 1 mm, whereas 1 mm HMA blocked 12 of 16 preparations. This pharmacology suggests low-affinity ENaCs are the important spindle mechanotransducer. In agreement with this, immunoreactivity to ENaC alpha, beta and gamma subunits was detected both by Western blot and immunocytochemistry. Immunofluorescence intensity ratios for ENaC alpha, beta or gamma relative to the vesicle marker synaptophysin in the same spindle all significantly exceeded controls (P < 0.001). Ratios for the related brain sodium channel ASIC2 (BNaC1alpha) were also highly significantly greater (P < 0.005). Analysis of confocal images showed strong colocalisation within the terminal of ENaC/ASIC2 subunits and synaptophysin. This study implicates ENaC and ASIC2 in mammalian mechanotransduction. Moreover, within the terminals they colocalise with synaptophysin, a marker for the synaptic-like vesicles which regulate afferent excitability in these mechanosensitive endings.

The delta-subunit of the epithelial sodium channel (ENaC) enhances channel activity and alters proteolytic ENaC activation

The epithelial sodium channel (ENaC) is probably a heterotrimer with three well characterized subunits (alphabetagamma). In humans an additional delta-subunit (delta-hENaC) exists but little is known about its function. Using the Xenopus laevis oocyte expression system, we compared the functional properties of alphabetagamma- and deltabetagamma-hENaC and investigated whether deltabetagamma-hENaC can be proteolytically activated. The amiloride-sensitive ENaC whole-cell current (DeltaI(ami)) was about 11-fold larger in oocytes expressing deltabetagamma-hENaC than in oocytes expressing alphabetagamma-hENaC. The 2-fold larger single-channel Na(+) conductance of deltabetagamma-hENaC cannot explain this difference. Using a chemiluminescence assay, we demonstrated that an increased channel surface expression is also not the cause. Thus, overall channel activity of deltabetagamma-hENaC must be higher than that of alphabetagamma-hENaC. Experiments exploiting the properties of the known betaS520C mutant ENaC confirmed this conclusion. Moreover, chymotrypsin had a reduced stimulatory effect on deltabetagamma-hENaC whole-cell currents compared with its effect on alphabetagamma-hENaC whole-cell currents (2-fold versus 5-fold). This suggests that the cell surface pool of so-called near-silent channels that can be proteolytically activated is smaller for deltabetagamma-hENaC than for alphabetagamma-hENaC. Proteolytic activation of deltabetagamma-hENaC was associated with the appearance of a delta-hENaC cleavage product at the cell surface. Finally, we demonstrated that a short inhibitory 13-mer peptide corresponding to a region of the extracellular loop of human alpha-ENaC inhibited DeltaI(ami) in oocytes expressing alphabetagamma-hENaC but not in those expressing deltabetagamma-hENaC. We conclude that the delta-subunit of ENaC alters proteolytic channel activation and enhances base-line channel activity.

Epithelial Na+ channel delta subunit is an acid sensor in the human oesophagus

Gastro-oesophageal reflux disease is caused by the reflux of gastric contents into the oesophagus, and thus the oesophageal lumen is damaged by gastric acid. The acid sensor involved in oesophageal epithelial defense is still unclear. Recently, we described that the epithelial Na(+) channel delta subunit (ENaCdelta) is a candidate molecule for a pH sensor in the human brain. Here, using reverse transcription-polymerase chain reaction and in situ hybridization methods, we showed that the proton-sensitive ENaCdelta was strongly expressed in the epithelial layer of the human oesophagus, representative peripheral tissue that can be exposed to an acidic environment. Other ENaC subunits (alpha, beta, and gamma) were also localized there. Based on the expression pattern, human oesophageal ENaC complex was mimicked in the Xenopus oocyte expression system and the response to acidic pH was recorded using a two-electrode voltage-clamp technique. The human oesophageal-mimicking ENaCdeltabetagammaalpha complex generated an amiloride-sensitive inward current at the holding potential of -60 mV. The ENaCdeltabetagammaalpha current was significantly activated by acidic pH (pH 4.0), approximately equal to the luminal value when gastric acid refluxes into the oesophagus. In conclusion, ENaCdelta is a candidate molecule for pH sensing in the gastrointestinal system in humans, providing a novel therapeutic target for gastro-oesophageal reflux disease.

Small molecule activator of the human epithelial sodium channel

The epithelial sodium channel (ENaC), a heterotrimeric complex composed of alpha, beta, and gamma subunits, belongs to the ENaC/degenerin family of ion channels and forms the principal route for apical Na(+) entry in many reabsorbing epithelia. Although high affinity ENaC blockers, including amiloride and derivatives, have been described, potent and specific small molecule ENaC activators have not been reported. Here we describe compound S3969 that fully and reversibly activates human ENaC (hENaC) in an amiloride-sensitive and dose-dependent manner in heterologous cells. Mechanistically, S3969 increases hENaC open probability through interactions requiring the extracellular domain of the beta subunit. hENaC activation by S3969 did not require cleavage by the furin protease, indicating that nonproteolyzed channels can be opened. Function of alphabetaG37Sgamma hENaC, a channel defective in gating that leads to the salt-wasting disease pseudohypoaldosteronism type I, was rescued by S3969. Small molecule activation of hENaC may find application in alleviating human disease, including pseudohypoaldosteronism type I, hypotension, and neonatal respiratory distress syndrome, when improved Na(+) flux across epithelial membranes is clinically desirable.

Expression analysis of the epithelial Na+ channel delta subunit in human melanoma G-361 cells

Malignant melanoma is the most deadly form of skin cancer and its incidence is steadily increasing worldwide. The plasma membrane in melanoma cells possesses a variety of ion channels, so its profile is thought to lead to a novel target for medical treatment for malignant melanoma. Here we showed that human melanoma G-361 cells expressed the epithelial Na(+) channel delta subunit (ENaC delta), which is largely unknown in physiological and pathological functions in non-neuronal tissues. Expression analyses at the level of mRNA clearly revealed that ENaC delta transcript was strongly expressed in human melanoma cells using reverse transcription-polymerase chain reaction and cell-based in situ hybridization techniques. Other ENaC subunits (alpha, beta, and gamma) were also distributed in human melanoma cells. In addition, human melanoma cells possessed an abundant expression of ENaC delta protein by immunocytochemistry. These results provide an attractive target for drug development of malignant melanoma.

Transient receptor potential channel TRPM8 agonists stimulate calcium influx and neurotensin secretion in neuroendocrine tumor cells

TRPM8 is a member of the melastatin-type transient receptor potential ion channel family. Activation by cold or by agonists (menthol, icilin) induces a transient rise in intracellular free calcium concentration ([Ca(2+)](i)). Our previous study demonstrated that Ca(2+)-permeable cation channels play a role in IGF-1-induced secretion of chromogranin A in human neuroendocrine tumor (NET) cell line BON [Mergler et al.: Neuroendocrinology 2006;82:87-102]. Here, we extend our earlier study by investigating the expression of TRPM8 and characterizing its impact on [Ca(2+)](i) and the secretion of neurotensin (NT). We identified TRPM8 expression in NET BON cells by RT-PCR, Western blotting and immunofluorescence staining. Icilin increased [Ca(2+)](i) in TRPM8-transfected human embryonic kidney cells (HEK293) but not in mock-transfected cells. Icilin and menthol induced Ca(2+) transients in BON cells as well as in primary NET cell cultures of two different pancreatic NETs as detected by single cell fluorescence imaging. Icilin increased non-selective cation channel currents in BON cells as detected by patch-clamp recordings. This activation was associated with increased NT secretion. Taken together, this study demonstrates for the first time the expression TRPM8 in NET cells and its role in regulating [Ca(2+)](i) and NT secretion. The regulation of NT secretion in NETs by TRPM8 may have a potential clinical implication in diagnosis or therapy.

A novel spliced variant of the epithelial Na+ channel δ-subunit in the human brain

The amiloride-sensitive epithelial Na+ channel regulates Na+ homeostasis in cells. Recently, we described that the delta-subunit is a candidate molecule for a pH sensor in the human brain. Here, an N-terminal spliced variant of the delta-subunit is cloned from human brain, and designated as the delta2-subunit, which is expressed with the original delta-subunit (delta1-subunit) at the same level in the human brain. Functional analyses revealed that the physiological and pharmacological properties (interaction with accessory betagamma-subunits, activation by acidic pH, amiloride sensitivity) of the delta2-subunit were similar to those of the delta1-subunit. In conclusion, the activities of both subunits may be involved in the mechanism underlying pH sensing in the human brain.

Evans blue is a specific antagonist of the human epithelial Na+ channel delta-subunit

The epithelial Na(+) channel (ENaC) regulates Na(+) homeostasis in cells and across epithelia. Four homologous ENaC subunits (alpha, beta, gamma, and delta) have been isolated in mammals. Combination of alpha-, beta-, and gamma-subunits or delta-, beta-, and gamma-subunits forms fully functional channels. Amiloride is a well known blocker of the ENaC family that inhibits both channel complexes. However, no specific antagonists are currently known that distinguish them. Here, we show that Evans blue, a diagnostic aid for the measurement of blood volume and vascular permeability, inhibits the activity of the delta-subunit expressed in Xenopus oocytes. The inward currents at a holding potential of -60 mV in human ENaCdeltabetagamma-expressing oocytes were inhibited by the application of Evans blue in a concentration-dependent manner with an IC(50) value of 143 muM. Evans blue markedly inhibited the delta-subunit current but did not block the alpha-subunit current. In conclusion, Evans blue is the first known delta-subunit-specific antagonist of ENaC. This finding provides us with a key compound for elucidating the physiological and pathological functions of ENaCdelta in humans and for drug development in the ENaC family.