Expression of the amiloride-sensitive sodium channel beta subunit gene in human B lymphocytes

Importance of Residual Water Permeability on the Excretion of Water during Water Diuresis in Rats

When the concentration of sodium (Na(+)) in arterial plasma (P(Na)) declines sufficiently to inhibit the release of vasopressin, water will be excreted promptly when the vast majority of aquaporin 2 water channels (AQP2) have been removed from luminal membranes of late distal nephron segments. In this setting, the volume of filtrate delivered distally sets the upper limit on the magnitude of the water diuresis. Since there is an unknown volume of water reabsorbed in the late distal nephron, our objective was to provide a quantitative assessment of this parameter. Accordingly, rats were given a large oral water load, while minimizing non-osmotic stimuli for the release of vasopressin. The composition of plasma and urine were measured. The renal papilla was excised during the water diuresis to assess the osmotic driving force for water reabsorption in the inner medullary collecting duct. During water diuresis, the concentration of creatinine in the urine was 13-fold higher than in plasma, which implies that ~8% of filtered water was excreted. The papillary interstitial osmolality was 600 mOsm/L > the urine osmolality. Since 17% of filtered water is delivered to the earliest distal convoluted tubule micropuncture site, we conclude that half of the water delivered to the late distal nephron is reabsorbed downstream during water diuresis. The enormous osmotic driving force for the reabsorption of water in the inner medullary collecting duct may play a role in this reabsorption of water. Possible clinical implications are illustrated in the discussion of a case example.

Sodium channels and multiple sclerosis: roles in symptom production, damage and therapy

Our understanding of the potential role of sodium channels in multiple sclerosis (MS) has grown substantially in recent years. The channels have long had a recognized role in the symptomatology of the disease, but now also have suspected roles in causing permanent axonal destruction, and a potential role in modulating the intensity of immune activity. Sodium channels might also provide an avenue to achieve axonal and neuronal protection in MS, thereby impeding the otherwise relentless advance of permanent neurological deficit. The symptoms of MS are largely determined by the conduction properties of axons and these, in turn, are largely determined by sodium channels. The number, subtype and distribution of the sodium channels are all important, together with the way that channel function is modified by local factors, such as those resulting from inflammation (eg, nitric oxide). Suspicion is growing that sodium channels may also contribute to the axonal degeneration primarily responsible for permanent neurological deficits. The proposed mechanism involves intra-axonal sodium accumulation which promotes reverse action of the sodium/calcium exchanger and thereby a lethal rise in intra-axonal calcium. Partial blockade of sodium channels protects axons from degeneration in experimental models of MS, and therapy based on this approach is currently under investigation in clinical trials. Some recent findings suggest that such systemic inhibition of sodium channels may also promote axonal protection by suppressing inflammation within the brain.

Genetic variation in the epithelial sodium channel: a risk factor for hypertension in people of african origin

High blood pressure occurs commonly in individuals of African origin, leading to an increased risk of cardiovascular and end-stage renal disease (ESRD). Black individuals frequently have low plasma renin activity, and their blood pressure responds well to salt reduction, suggesting that abnormalities in renal sodium handling may be important in the etiology of hypertension in this population. The epithelial sodium channel (ENaC) has a central role in sodium transport across membranes, and in the kidney it contributes to the regulation of blood pressure via changes in sodium balance and blood volume. Rare monogenetic disorders have been described in association with hypertension, such as Liddle's syndrome. In addition, other ENaC polymorphisms have also been described, some of which are more common in black individuals. The T594M polymorphism of ENaC occurs exclusively in black individuals and is associated with hypertension in a black South London population. There is preliminary evidence that amiloride is effective as monotherapy in hypertensives with the T594M polymorphism, and a further study is underway to determine whether this is indeed a safe and specific treatment. If so, then amiloride may provide an important new strategy for blood pressure control in affected black hypertensives.

Amiloride-sensitive epithelial sodium channel subunits are expressed in human and mussel immunocytes

In this study, we examined the expression of epithelial Na(+) channel (ENaC) subunits in human peripheral blood lymphocytes, human lymph nodes and molluscan immunocytes using non-radioactive in situ hybridization. The results showed that T lymphocytes express the ENaC gamma subunit mRNA, and B lymphocytes the ENaC beta subunit mRNA. Yet, the alpha subunit mRNA was not detected in either cell type. In molluscan immunocytes, all three homologous ENaC subunit mRNAs are present, and these data were also confirmed by RT-PCR and sequencing of the PCR products. These findings show evolutionary conservation of the expression of ENaC subunits in immunocytes of invertebrates to vertebrates. The observed differential expression patterns of ENaC subunits suggest that ENaC function may be regulated differentially in different types of human lymphocytes.

The epithelial Na+ channel: cell surface insertion and retrieval in Na+ homeostasis and hypertension

The epithelial Na+ channel (ENaC) forms the pathway for Na+ absorption in the kidney collecting duct and other epithelia. Dominant gain-of-function mutations cause Liddle's syndrome, an inherited form of hypertension resulting from excessive renal Na+ absorption. Conversely, loss-of-function mutations cause pseudohypoaldosteronism type I, a disorder of salt wasting and hypotension. Thus, ENaC has a critical role in the maintenance of Na+ homeostasis and blood pressure control. Altered Na+ absorption in the lung may also contribute to the pathogenesis of cystic fibrosis. Epithelial Na+ absorption is regulated in large part by mechanisms that control the expression of ENaC at the cell surface. Nedd4, a ubiquitin protein ligase, binds to ENaC and targets the channel for endocytosis and degradation. Liddle's syndrome mutations disrupt the interaction between ENaC and Nedd4, resulting in an increase in the number of ENaC channels at the cell surface. Aldosterone and vasopressin also regulate Na+ absorption to defend against hypotension and hypovolemia. Both hormones increase the expression of ENaC at the cell surface. The goal of this review is to summarize recent data on the regulation of ENaC expression at the cell surface.

Molecular basis for the cross-reactivity of antibodies elicited by a natural anatoxin with alpha- and beta-toxins from the venom of Tityus serrulatus scorpion

A non-toxic protein (TsNTxP) isolated from the venom of the noxious scorpion Tityus serrulatus (Ts) induces polyclonal antibodies cross-reactive with several toxins from the venom, in sharp contrast to anti-toxin antibodies which are toxin specific. To try to uncover the molecular basis for these unusual properties, peptide scanning experiments were performed and indicated that the N- and C-terminal parts of TsNTxP enclose continuous epitopes (residues 1-15 and 47-61). Antibodies raised against peptides corresponding to these two regions were found to have neutralizing properties against a mixture of all toxic proteins from the T. serrulatus venom, indicating that residues 1-15 and 47-61 correspond to neutralizing epitopes. The identification of key antigenic residues within these two epitopes revealed that several of them are well conserved in the amino-acid sequences of the three main toxins (Ts II, Ts IV and Ts VII) from the venom: Glu 3, Tyr 5, Asp 8, Asp 50, Trp 55 and Lys 61. A single key-residue (Glu 58) is unique to TsNTxP. By using homology modeling, a model of the three-dimensional structure of TsNTxP was obtained. The antigenically important residues from TsNTxP were found to be surface exposed, with five of them clustered on the facet of the protein reported to enclose the active site of toxins. Residues equivalent to the seven key-residues of the anatoxin were also found to be exposed in the active toxins from T. serrulatus venom. These results show that antibodies elicited by the non-toxic protein TsNTxP recognized, within the N- and C-terminal parts of toxins of T. serrulatus, conserved and surface exposed residues which might also be involved in the toxic action of the proteins.

An Alu cassette in the human epithelial sodium channel

Here, we report the presence of two splice variants of the human epithelial sodium channel alpha subunit (h alpha ENaC) containing Alu cassette, namely h alpha ENaC+22 and h alpha ENaC+Alu, in various tissues. Functional expression of these splice variants with hENaC beta and gamma subunits produced loss-of-channel activity in the Xenopus oocyte expression system. Interestingly, coexpression of h alpha ENaC+22 or h alpha ENaC+Alu, respectively, with wild type hENaC alpha, beta, and gamma subunits enhanced the expression of amiloride-sensitive current in oocytes. The presence of Alu sequences in the 3'-untranslated region of h gamma ENaC was also identified.

Location and function of the epithelial Na channel in the cochlea

In the cochlea, endolymph is a K-rich and Na-poor fluid. The purpose of the present study was to check the presence and to assess the role of epithelial Na channel (ENaC) in this organ. alpha-, beta-, and gamma-ENaC subunit mRNA, and proteins were detected in rat cochlea by RT-PCR and Western blot. alpha-ENaC subunit mRNA was localized by in situ hybridization in both epithelial (stria vascularis, spiral prominence, spiral limbus) and nonepithelial structures (spiral ligament, spiral ganglion). The alpha-ENaC-positive tissues were also positive for beta-subunit mRNA (except spiral ganglion) or for gamma-subunit mRNA (spiral limbus, spiral ligament, and spiral ganglion), but the signals of beta- and gamma-subunits were weaker than those observed for alpha-subunit. In vivo, the endocochlear potential was recorded in guinea pigs under normoxic and hypoxic conditions after endolymphatic perfusion of ENaC inhibitors (amiloride, benzamil) dissolved either in K-rich or Na-rich solutions. ENaC inhibitors altered the endocochlear potential when Na-rich but not when K-rich solutions were perfused. In conclusion, ENaC subunits are expressed in epithelial and nonepithelial cochlear structures. One of its functions is probably to maintain the low concentration of Na in endolymph.

Aldosterone-related genetic effects in hypertension

Our basic understanding of Na(+) transport mechanisms provides unique insights into epithelial transport processes. Unusual clinical syndromes can arise from mutations of these ion transporters. These genetic disorders affect Na(+) balance, resulting in both N(a+) retaining and Na(+) wasting conditions. A major focus has been the epithelial sodium channel (ENaC), which can be activated by mutations (eg, Liddle's syndrome), changes in the response to mineralocorticoids (apparent mineralocorticoid excess syndrome), or production of mineralocorticoids (glucocorticoid-remediable aldosteronism). As a result, we now have clearly defined Mendelian syndromes in which ENaC activity is "dysregulated." This dysregulation leads to systemic hypertension associated with suppressed plasma renin activity, which can be attributed to a primary renal mechanism. Applying these insights to the far more common disorder of low-renin hypertension may shed new light on the underlying pathophysiology of this common form of human hypertension.

General overview of mineralocorticoid hormone action

The adrenal cortex elaborates two major groups of steroids that have been arbitrarily classified as glucocorticoids and mineralocorticoids, despite the fact that carbohydrate metabolism is intimately linked to mineral balance in mammals. In fact, glucocorticoids assured both of these functions in all living cells, animal and photosynthetic, prior to the appearance of aldosterone in teleosts at the dawn of terrestrial colonization. The evolutionary drive for a hormone specifically designed for hydromineral regulation led to zonation for the conversion of 18-hydroxycorticosterone into aldosterone through the catalytic action of a synthase in the secluded compartment of the adrenal zona glomerulosa. Corticoid hormones exert their physiological action by binding to receptors that belong to a transcription factor superfamily, which also includes some of the proteins regulating steroid synthesis. Steroids stimulate sodium absorption by the activation and/or de novo synthesis of the ion-gated, amiloride-sensitive sodium channel in the apical membrane and that of the Na+/K+-ATPase in the basolateral membrane. Receptors, channels, and pumps apparently are linked to the cytoskeleton and are further regulated variously by methylation, phosphorylation, ubiquination, and glycosylation, suggesting a complex system of control at multiple checkpoints. Mutations in genes for many of these different proteins have been described and are known to cause clinical disease.

Genetic analysis of the beta subunit of the epithelial Na+ channel in essential hypertension

Mutations of the last exon of the beta subunit of the amiloride-sensitive epithelial Na+ channel (betaENaC) can lead to Liddle's syndrome, a rare monogenic form of hypertension. The objective of this study was to test whether more subtle changes of betaENaC could be implicated in essential hypertension. After determination of the betaENaC coding gene organization (12 exons spanning 23.5 kb), a systematic screening of the last exon of the gene was performed in 525 subjects (475 whites, 50 Afro-Caribbeans), all probands of hypertensive families. This search was extended to the remaining 11 exons in a subset of 101 probands with low-renin hypertension. Seven amino acid changes were detected: G589S, T594M, R597H, R624C, E632G (last exon), G442V, and V434M (exon 8). These genetic variants were more frequent in subjects of African origin (44%) than in whites (1%). The functional properties of the variants were analyzed in Xenopus oocytes by two independent techniques, ie, electrophysiology and 22Na+ uptake. Small but not significant differences were observed between the variants and wild type. The clinical evaluation of the family bearing the G589S variant, which provided the highest relative ENaC activity, did not show a cosegregation between the mutation and hypertension. The present study illustrates the difficulty in establishing a relation of causality between a susceptibility gene and hypertension. Furthermore, it does not favor a substantial role of the betaENaC gene in essential hypertension.

Cloning and functional studies of splice variants of the alpha-subunit of the amiloride-sensitive Na+ channel

The alpha-subunit of the amiloride-sensitive epithelial Na+ channel (alpha ENaC) is critical in forming an ion conductive pore in the membrane. We have identified the wild-type and three splice variants of the human alpha ENaC (h alpha ENaC) from the human lung cell line H441, using RT-PCR. These splice variants contain various structures in the extracellular domain, resulting in premature truncation (h alpha ENaCx), 19-amino acid deletion (h alpha ENaC-19), and 22-amino acid insertion (h alpha ENaC + 22). Wild-type h alpha ENaC and splice variants were functionally characterized in Xenopus oocytes by coexpression with hENaC beta- and gamma-subunits. Unlike wild-type h alpha ENaC, undetectable or substantially reduced amiloride-sensitive currents were observed in oocytes expressing these splice variants. Wild-type h alpha ENaC was the most abundantly expressed h alpha ENaC mRNA species in all tissues in which its expression was detected. These findings indicate that the extracellular domain is important to generate structural and functional diversity of h alpha ENaC and that alternative splicing may play a role in regulating hENaC activity.

Loss of protein kinase C inhibition in the beta-T594M variant of the amiloride-sensitive Na+ channel

We previously reported the presence of a novel variant (beta-T594M) of the amiloride-sensitive Na+ channel (ASSC) in which the threonine residue at position 594 in the beta-subunit has been replaced by a methionine residue. Electrophysiological studies of the ASSC on Epstein-Barr virus (EBV)-transformed lymphocytes carrying this variant showed that the 8-(4-chlorophenylthio) adenosine 3':5'-cyclic monophosphate (8cpt-cAMP)-induced responses were enhanced when compared to wild-type EBV-transformed lymphocytes. Furthermore, in wild-type EBV-transformed cells, the 8cpt-cAMP-induced response was totally blocked by the phorbol ester, phorbol 12-myristate 13-acetate (PMA). This inhibitory effect of PMA was blocked by a protein kinase C inhibitor, chelerythrine. We now have identified individuals who are homozygous for this variant, and showed that PMA had no effect on the 8cpt-cAMP-induced responses in the EBV-transformed lymphocytes from such individuals. Cells heterozygous for this variant showed mixed responses to PMA, with the majority of cells partially inhibited by PMA. Our results demonstrate that an alteration in a single amino acid residue in the beta-subunit of the ASSC can lead to a total loss of inhibition to PMA, and establish the beta-subunit as having an important role in conferring a regulatory effect on the ASSC of lymphocytes.