Knock Out of CGN and CGNL1 in MDCK Cells Affects Claudin-2 but Has a Minor Impact on Tight Junction Barrier Function.

Cingulin (CGN) and paracingulin (CGNL1) are cytoplasmic proteins of tight junctions (TJs), where they play a role in tethering ZO-1 to the actomyosin and microtubule cytoskeletons. The role of CGN and CGNL1 in the barrier function of epithelia is not completely understood. Here, we analyzed the effect of the knock out (KO) of either CGN or CGNL1 or both on the paracellular permeability of monolayers of kidney epithelial (MDCK) cells. KO cells displayed a modest but significant increase in the transepithelial resistance (TER) of monolayers both in the steady state and during junction assembly by the calcium switch, whereas the permeability of the monolayers to 3 kDa dextran was not affected. The permeability to sodium was slightly but significantly decreased in KO cells. This phenotype correlated with slightly increased mRNA levels of claudin-2, slightly decreased protein levels of claudin-2, and reduced junctional accumulation of claudin-2, which was rescued by CGN or CGNL1 but not by ZO-1 overexpression. These results confirm previous observations indicating that CGN and CGNL1 are dispensable for the barrier function of epithelia and suggest that the increase in the TER in clonal lines of MDCK cells KO for CGN, CGNL1, or both is due to reduced protein expression and junctional accumulation of the sodium pore-forming claudin, claudin-2.

Renal water transport in health and disease.

Saving body water by optimal reabsorption of water filtered by the kidney leading to excretion of urine with concentrations of solutes largely above that of plasma allowed vertebrate species to leave the aquatic environment to live on solid ground. Filtered water is reabsorbed for 70% and 20% by proximal tubules and thin descending limbs of Henle, respectively. These two nephron segments express the water channel aquaporin-1 located along both apical and basolateral membranes. In the proximal tubule, the paracellular pathway accounts for at least 30% of water reabsorption, and the tight-junction core protein claudin-2 plays a key role in this permeability. The ascending limb of Henle and the distal convoluted tubule are impermeant to water and are responsible for urine dilution. The water balance is adjusted along the collecting system, i.e. connecting tubule and the collecting duct, under the control of arginine-vasopressin (AVP). AVP is synthesized by the hypothalamus and released in response to an increase in extracellular osmolality or stimulation of baroreceptors by decreased blood pressure. In response to AVP, aquaporin-2 water channels stored in subapical intracellular vesicles are translocated to the apical plasma membrane and raise the water permeability of the collecting system. The basolateral step of water reabsorption is mediated by aquaporin-3 and -4, which are constitutively expressed. Drugs targeting water transport include classical diuretics, which primarily inhibit sodium transport; the new class of SGLT2 inhibitors, which promotes osmotic diuresis and the non-peptidic antagonists of the V2 receptor, which are pure aquaretic drugs. Disturbed water balance includes diabetes insipidus and hyponatremias. Diabetes insipidus is characterized by polyuria and polydipsia. It is either related to a deficit in AVP secretion called central diabetes insipidus that can be treated by AVP analogs or to a peripheral defect in AVP response called nephrogenic diabetes insipidus. Diabetes insipidus can be either of genetic origin or acquired. Hyponatremia is a common disorder most often related to free water excess relying on overstimulated or inappropriate AVP secretion. The assessment of blood volume is key for the diagnosis and treatment of hyponatremia, which can be classified as hypo-, eu-, or hypervolemic.

Systemic bis-phosphinic acid derivative restores chloride transport in Cystic Fibrosis mice.

Mutations in the Cystic Fibrosis Transmembrane Conductance Regulator gene (CFTR) are responsible for Cystic Fibrosis (CF). The most common CF-causing mutation is the deletion of the 508th amino-acid of CFTR (F508del), leading to dysregulation of the epithelial fluid transport in the airway's epithelium and the production of a thickened mucus favoring chronic bacterial colonization, sustained inflammation and ultimately respiratory failure. c407 is a bis-phosphinic acid derivative which corrects CFTR dysfunction in epithelial cells carrying the F508del mutation. This study aimed to investigate c407 in vivo activity in the F508del Cftrtm1Eur murine model of CF. Using nasal potential difference measurement, we showed that in vivo administration of c407 by topical, short-term intraperitoneal and long-term subcutaneous route significantly increased the CFTR dependent chloride (Cl-) conductance in F508del Cftrtm1Eur mice. This functional improvement was correlated with a relocalization of F508del-cftr to the apical membrane in nasal epithelial cells. Importantly, c407 long-term administration was well tolerated and in vitro ADME toxicologic studies did not evidence any obvious issue. Our data provide the first in vivo preclinical evidence of c407 efficacy and absence of toxicity after systemic administration for the treatment of Cystic Fibrosis.

Activation of the Hypoxia-Inducible Factor Pathway Inhibits Epithelial Sodium Channel-Mediated Sodium Transport in Collecting Duct Principal Cells.

BACKGROUND: Active sodium reabsorption is the major factor influencing renal oxygen consumption and production of reactive oxygen species (ROS). Increased sodium reabsorption uses more oxygen, which may worsen medullary hypoxia and produce more ROS via enhanced mitochondrial ATP synthesis. Both mechanisms may activate the hypoxia-inducible factor (HIF) pathway. Because the collecting duct is exposed to low oxygen pressure and variations of active sodium transport, we assessed whether the HIF pathway controls epithelial sodium channel (ENaC)-dependent sodium transport. METHODS: We investigated HIF's effect on ENaC expression in mpkCCD cl4 cells (a model of collecting duct principal cells) using real-time PCR and western blot and ENaC activity by measuring amiloride-sensitive current. We also assessed the effect of hypoxia and sodium intake on abundance of kidney sodium transporters in wild-type and inducible kidney tubule-specific Hif1α knockout mice. RESULTS: In cultured cells, activation of the HIF pathway by dimethyloxalylglycine or hypoxia inhibited sodium transport and decreased expression of ß ENaC and γ ENaC, as well as of Na,K-ATPase. HIF1 α silencing increased ß ENaC and γ ENaC expression and stimulated sodium transport. A constitutively active mutant of HIF1 α produced the opposite effect. Aldosterone and inhibition of the mitochondrial respiratory chain slowly activated the HIF pathway, suggesting that ROS may also activate HIF. Decreased γ ENaC abundance induced by hypoxia in normal mice was abolished in Hif1α knockout mice. Similarly, Hif1α knockout led to increased γ ENaC abundance under high sodium intake. CONCLUSIONS: This study reveals that γ ENaC expression and activity are physiologically controlled by the HIF pathway, which may represent a negative feedback mechanism to preserve oxygenation and/or prevent excessive ROS generation under increased sodium transport.

Expression of claudin-8 is induced by aldosterone in renal collecting duct principal cells.

Fine tuning of Na+ reabsorption takes place along the aldosterone-sensitive distal nephron, which includes the collecting duct (CD), where it is mainly regulated by aldosterone. In the CD, Na+ reabsorption is mediated by the epithelial Na+ channel and Na+ pump (Na+-K+-ATPase). Paracellular ion permeability is mainly dependent on tight junction permeability. Claudin-8 is one of the main tight junction proteins expressed along the aldosterone-sensitive distal nephron. We have previously shown a coupling between transcellular Na+ reabsorption and paracellular Na+ barrier. We hypothesized that aldosterone controls the expression levels of both transcellular Na+ transporters and paracellular claudin-8 in a coordinated manner. Here, we show that aldosterone increased mRNA and protein levels as well as lateral membrane localization of claudin-8 in cultured CD principal cells. The increase in claudin-8 mRNA levels in response to aldosterone was prevented by preincubation with 17-hydroxyprogesterone, a mineralocorticoid receptor antagonist, and by inhibition of transcription with actinomycin D. We also showed that a low-salt diet, which stimulated aldosterone secretion, was associated with increased claudin-8 abundance in the mouse kidney. Reciprocally, mice subjected to a high-salt diet, which inhibits aldosterone secretion, or treated with spironolactone, a mineralocorticoid receptor antagonist, displayed decreased claudin-8 expression. Inhibition of glycogen synthase kinase-3, Lyn, and Abl signaling pathways prevented the effect of aldosterone on claudin-8 mRNA and protein abundance, suggesting that signaling of protein kinases plays a permissive role on the transcriptional activity of the mineralocorticoid receptor. This study shows that signaling via multiple protein kinases working in concert mediates aldosterone-induced claudin-8 expression in the CD.NEW & NOTEWORTHY In this study, we showed that aldosterone modulates claudin-8 expression in cultured collecting duct principal cells and in the mouse kidney. The upregulation of claudin-8 expression in response to aldosterone is dependent on at least glycogen synthase kinase-3, Lyn, and Abl signaling pathways, indicating the participation of multiple protein kinases to the effect of aldosterone.

A variant of ASIC2 mediates sodium retention in nephrotic syndrome.

Idiopathic nephrotic syndrome (INS) is characterized by proteinuria and renal sodium retention leading to edema. This sodium retention is usually attributed to epithelial sodium channel (ENaC) activation after plasma aldosterone increase. However, most nephrotic patients show normal aldosterone levels. Using a corticosteroid-clamped (CC) rat model of INS (CC-PAN), we showed that the observed electrogenic and amiloride-sensitive Na retention could not be attributed to ENaC. We then identified a truncated variant of acid-sensing ion channel 2b (ASIC2b) that induced sustained acid-stimulated sodium currents when coexpressed with ASIC2a. Interestingly, CC-PAN nephrotic ASIC2b-null rats did not develop sodium retention. We finally showed that the expression of the truncated ASIC2b in the kidney was dependent on the presence of albumin in the tubule lumen and activation of ERK in renal cells. Finally, the presence of ASIC2 mRNA was also detected in kidney biopsies from patients with INS but not in any of the patients with other renal diseases. We have therefore identified a variant of ASIC2b responsible for the renal Na retention in the pathological context of INS.

Interaction between Epithelial Sodium Channel γ-Subunit and Claudin-8 Modulates Paracellular Sodium Permeability in Renal Collecting Duct.

BACKGROUND: Water and solute transport across epithelia can occur via the transcellular or paracellular pathways. Tight junctions play a key role in mediating paracellular ion reabsorption in the kidney. In the renal collecting duct, which is a typical absorptive tight epithelium, coordination between transcellular sodium reabsorption and paracellular permeability may prevent the backflow of reabsorbed sodium to the tubular lumen along a steep electrochemical gradient. METHODS: To investigate whether transcellular sodium transport controls tight-junction composition and paracellular permeability via modulating expression of the transmembrane protein claudin-8, we used cultured mouse cortical collecting duct cells to see how overexpression or silencing of epithelial sodium channel (ENaC) subunits and claudin-8 affect paracellular permeability. We also used conditional kidney tubule-specific knockout mice lacking ENaC subunits to assess the ENaC's effect on claudin-8 expression. RESULTS: Overexpression or silencing of the ENaC γ-subunit was associated with parallel and specific changes in claudin-8 abundance. Increased claudin-8 abundance was associated with a reduction in paracellular permeability to sodium, whereas decreased claudin-8 abundance was associated with the opposite effect. Claudin-8 overexpression and silencing reproduced these functional effects on paracellular ion permeability. Conditional kidney tubule-specific ENaC γ-subunit knockout mice displayed decreased claudin-8 expression, confirming the cell culture experiments' findings. Importantly, ENaC ß-subunit or α-subunit silencing or kidney tubule-specific ß-ENaC or α-ENaC knockout mice did not alter claudin-8 abundance. CONCLUSIONS: Our data reveal the specific coupling between ENaC γ-subunit and claudin-8 expression. This coupling may play an important role in preventing the backflow of reabsorbed solutes and water to the tubular lumen, as well as in coupling paracellular and transcellular sodium permeability.

Time-course of sodium transport along the nephron in nephrotic syndrome: The role of potassium.

The mechanism of sodium retention and its location in kidney tubules may vary with time in nephrotic syndrome (NS). We studied the mechanisms of sodium retention in transgenic POD-ATTAC mice, which display an inducible podocyte-specific apoptosis. At day 2 after the induction of NS, the increased abundance of NHE3 and phosphorylated NCC in nephrotic mice compared with controls suggest that early sodium retention occurs mainly in the proximal and distal tubules. At day 3, the abundance of NHE3 normalized, phosphorylated NCC levels decreased, and cleavage and apical localization of γ-ENaC increased in nephrotic mice. These findings indicate that sodium retention shifted from the proximal and distal tubules to the collecting system. Increased cleavage and apical localization of γ-ENaC persisted at day 5 in nephrotic mice when hypovolemia resolved and steady-state was reached. Sodium retention and γ-ENaC cleavage were independent of the increased plasma levels of aldosterone. Nephrotic mice displayed decreased glomerular filtration rate and urinary potassium excretion associated with hyperkaliemia at day 3. Feeding nephrotic mice with a low potassium diet prevented hyperkaliemia, γ-ENaC cleavage, and led to persistent increased phosphorylation of NCC. These results suggest that potassium homeostasis is a major determinant of the tubular site of sodium retention in nephrotic mice.

Resolvin D1 regulates epithelial ion transport and inflammation in cystic fibrosis airways.

BACKGROUND: Cystic Fibrosis (CF) lung disease is characterised by dysregulated ion transport that promotes chronic bacterial infection and inflammation. The impact of the specialised pro-resolution mediator resolvin D1 (RvD1) on airway surface liquid (ASL) dynamics and innate defence had not yet been investigated in CF airways. METHODS: Ex vivo studies were performed on primary cultures of alveolar macrophages and bronchial epithelial cells from children with CF and in human bronchial epithelial cell lines; in vivo studies were performed in homozygous F508del-CFTR mice treated with vehicle control or RvD1 (1-100nM). RESULTS: RvD1 increased the CF ASL height in human bronchial epithelium and restored the nasal trans-epithelial potential difference in CF mice by decreasing the amiloride-sensitive Na+ absorption and stimulating CFTR-independent Cl- secretion. RvD1 decreased TNFα induced IL-8 secretion and enhanced the phagocytic and bacterial killing capacity of human CF alveolar macrophages. CONCLUSION: RvD1 resolves CF airway pathogenesis and has therapeutic potential in CF lung disease.

Analysis of nasal potential in murine cystic fibrosis models.

The nasal epithelium of the mouse closely mimics the bioelectrical phenotype of the human airways. Ion transport across the nasal epithelium induces a nasal transepithelial potential difference. Its measurement by a relatively non-invasive method adapted from humans allows in vivo longitudinal measurements of CFTR-dependent ionic transport in the murine nasal mucosa. This test offers a useful tool to assess CFTR function in preclinical studies for novel therapeutics modulating CFTR activity. Here we extensively review work done to assess transepithelial transport in the murine respiratory epithelium in the basal state and after administration of CFTR modulators. Factors of variability and discriminative threshold between the CF and the WT mice for different readouts are discussed.

The rice monovalent cation transporter OsHKT2;4: revisited ionic selectivity.

The family of plant membrane transporters named HKT (for high-affinity K(+) transporters) can be subdivided into subfamilies 1 and 2, which, respectively, comprise Na(+)-selective transporters and transporters able to function as Na(+)-K(+) symporters, at least when expressed in yeast (Saccharomyces cerevisiae) or Xenopus oocytes. Surprisingly, a subfamily 2 member from rice (Oryza sativa), OsHKT2;4, has been proposed to form cation/K(+) channels or transporters permeable to Ca(2+) when expressed in Xenopus oocytes. Here, OsHKT2;4 functional properties were reassessed in Xenopus oocytes. A Ca(2+) permeability through OsHKT2;4 was not detected, even at very low external K(+) concentration, as shown by highly negative OsHKT2;4 zero-current potential in high Ca(2+) conditions and lack of sensitivity of OsHKT2;4 zero-current potential and conductance to external Ca(2+). The Ca(2+) permeability previously attributed to OsHKT2;4 probably resulted from activation of an endogenous oocyte conductance. OsHKT2;4 displayed a high permeability to K(+) compared with that to Na(+) (permeability sequence: K(+) > Rb(+) ≈ Cs(+) > Na(+) ≈ Li(+) ≈ NH(4)(+)). Examination of OsHKT2;4 current sensitivity to external pH suggested that H(+) is not significantly permeant through OsHKT2;4 in most physiological ionic conditions. Further analyses in media containing both Na(+) and K(+) indicated that OsHKT2;4 functions as K(+)-selective transporter at low external Na(+), but transports also Na(+) at high (>10 mm) Na(+) concentrations. These data identify OsHKT2;4 as a new functional type in the K(+) and Na(+)-permeable HKT transporter subfamily. Furthermore, the high permeability to K(+) in OsHKT2;4 supports the hypothesis that this system is dedicated to K(+) transport in the plant.