Activating FcγR function depends on endosomal-signaling platforms

Cell surface receptor internalization can either terminate signaling or activate alternative endosomal signaling pathways. We investigated here whether endosomal signaling is involved in the function of the human receptors for Fc immunoglobulin fragments (FcRs): FcαRI, FcγRIIA, and FcγRI. All these receptors were internalized after their cross-linking with receptor-specific antibodies, but their intracellular trafficking was different. FcαRI was targeted directly to lysosomes, while FcγRIIA and FcγRI were internalized in particular endosomal compartments described by the insulin esponsive minoeptidase (IRAP), where they recruited signaling molecules, such as the active form of the kinase Syk, PLCγ and the adaptor LAT. Destabilization of FcγR endosomal signaling in the absence of IRAP compromised cytokine secretion downstream FcγR activation and macrophage ability to kill tumor cells by antibody-dependent cell-mediated cytotoxicity (ADCC). Our results indicate that FcγR endosomal signaling is required for the FcγR-driven inflammatory reaction and possibly for the therapeutic action of monoclonal antibodies.

LC3-associated phagocytosis protects against inflammation and liver fibrosis via immunoreceptor inhibitory signaling

Sustained hepatic and systemic inflammation, particularly originating from monocytes/macrophages, is a driving force for fibrosis progression to end-stage cirrhosis and underlies the development of multiorgan failure. Reprogramming monocyte/macrophage phenotype has emerged as a strategy to limit inflammation during chronic liver injury. Here, we report that LC3-associated phagocytosis (LAP), a noncanonical form of autophagy, protects against hepatic and systemic inflammation during chronic liver injury in rodents, with beneficial antifibrogenic effects. LAP is enhanced in blood and liver monocytes from patients with fibrosis and cirrhosis. Pharmacological inhibition of LAP components in human monocytes from patients with cirrhosis or genetic disruption of LAP in mice with chronic liver injury exacerbates both the inflammatory signature in isolated human monocytes and the hepatic inflammatory profile in mice, resulting in enhanced liver fibrosis. Mechanistically, patients with cirrhosis showed increased monocyte expression of Fc fragment of IgG receptor IIA (FcγRIIA) and enhanced engulfment of immunoglobulin G in LC3⁺ phagosomes that triggers an FcγRIIA/Src homology region 2 domain-containing phosphatase-1 (SHP-1) inhibitory immunoreceptor tyrosine-based activation motif (ITAMi) anti-inflammatory pathway. Mice overexpressing human FcγRIIA in myeloid cells show enhanced LAP in response to chronic liver injury and resistance to inflammation and liver fibrosis. Activation of LAP is lost in monocytes from patients with multiorgan failure and restored by specifically targeting ITAMi signaling with anti-FcγRIIA F(ab')₂ fragments, or with intravenous immunoglobulin (IVIg). These data suggest the existence of an ITAMi-mediated mechanism by which LAP might protect against inflammation. Sustaining LAP may open therapeutic perspectives for patients with chronic liver disease.

LC3-associated phagocytosis in myeloid cells, a fireman that restrains inflammation and liver fibrosis, via immunoreceptor inhibitory signaling

Control of systemic and hepatic inflammation, in particular originating from monocytes/macrophages, is crucial to prevent liver fibrosis and its progression to end-stage cirrhosis. LC3-associated phagocytosis (LAP) is a non-canonical form of autophagy that shifts the monocyte/macrophage phenotype to an anti-inflammatory phenotype. In a recent study, we uncovered LAP as a protective mechanism against inflammation-driven liver fibrosis and systemic inflammation in the context of cirrhosis. We observed that LAP is enhanced in blood and liver monocytes from patients with liver fibrosis or those who progress to cirrhosis. Combining studies in which LAP was pharmacologically or genetically inactivated, we found that LAP limits inflammation in monocytes from cirrhotic patients, and the hepatic inflammatory profile in mice with chronic liver injury, resulting in anti-fibrogenic effects. Mechanistically, LAP-induced anti-inflammatory and antifibrogenic signaling results from enhanced expression of the Fc immunoreceptor FCGR2A/FcγRIIA and activation of an FCGR2A-mediated PTPN6/SHP-1 anti-inflammatory pathway, leading to increased engulfment of IgG into LC3 ⁺ phagosomes. In patients with cirrhosis progressing to multi-organ failure (acute-on chronic liver failure), LAP is lost in monocytes, and can be restored by targeting FCGR2A-mediated PTPN6/SHP-1 signaling. These data suggest that sustaining LAP may open novel therapeutic perspectives for patients with end-stage liver disease.

Influence of Polymorphonuclear Leukocytes on the Metabolism of Arachidonate in Human Platelets

The effect of the association of purified polymorphonuclear leukocytes (PMNL) with platelets on arachidonic acid (AA) metabolism was studied in the presence of various concentrations of this fatty acid. Both thromboxane B2 (TXB2) and 12-hydroxyeicosatetraenoic acid (12-HETE) were measured. In the presence of tracer doses of AA, addition of increasing amounts of PMNL to platelets inhibited in a concentration-dependent manner their 12-HETE and TXB2 production. This inhibition was not due to diversion of AA metabolism towards other pathways since, apart a negligible amount of 12,20-diHETE, no other product could be detected. Inhibition of piatelet-TXB2 synthesis by PMNL persisted at increasing concentrations of AA below 16 μM. Above this concentration, TXB2 production by platelets incubated alone diminished progressively. Addition of PMNL blunted in part this inhibitory effect and even resulted, above 16 μM AA, in an increased production of TXB2. In contrast with what was observed for TXB2 formation, the inhibition of 12-HETE synthesis persisted when PMNL and platelets were coincubated in the presence of high doses of AA (163 μM). At this concentration, 15-HETE generation became apparent for each cell type incubated separately and was markedly enhanced in the coincubation studies. The present investigation demonstrates that the presence of PMNL modifies the metabolism of arachidonate by human platelets. Moreover, this cell-cell interaction markedly depends on the concentration of substrate. PMNL in excess may attenuate synthesis by platelets of their toxic products.

Cyclosporine A Induces MicroRNAs Controlling Innate Immunity during Renal Bacterial Infection

Urinary tract infections (UTIs) mainly due to uropathogenic Escherichia coli (UPEC) are one of the most frequent complications in kidney-transplanted patients, causing significant morbidity. However, the mechanisms underlying UTI in renal grafts remain poorly understood. Here, we analysed the effects of the potent immunosuppressive agent cyclosporine A (CsA) on the activation of collecting duct cells that represent a preferential site of adhesion and translocation for UPEC. CsA induced the inhibition of lipopolysaccharide- induced activation of collecting duct cells due to the downregulation of the expression of TLR4 via the microRNA Let-7i. Using an experimental model of ascending UTI, we showed that the pretreatment of mice with CsA prior to infection induced a marked fall in cytokine production by collecting duct cells, neutrophil recruitment, and a dramatic rise of bacterial load, but not in infected TLR4-defective mice kidneys. This effect was also observed in CsA-treated infected kidneys, where the expression of Let-7i was increased. Treatment with a synthetic Let-7i mimic reproduced the effects of CsA. Conversely, pretreatment with an anti-Let-7i antagonised the effects of CsA and rescued the innate immune response of collecting duct cells against UPEC. Thus, the utilisation of an anti-Let-7i during kidney transplantation may protect CsA-treated patients from ascending bacterial infection.

Flagellin/TLR5 signalling activates renal collecting duct cells and facilitates invasion and cellular translocation of uropathogenic Escherichia coli

Uropathogenic Escherichia coli (UPEC) colonizing kidneys is the main cause of acute pyelonephritis. TLR5 that senses flagellin was shown to be highly expressed in the bladder and to participate in host defence against flagellated UPEC, although its role in kidneys still remains elusive. Here we show that TLR5 is expressed in renal medullary collecting duct (MCD) cells, which represent a preferential site of UPEC adhesion. Flagellin, like lipopolysaccharide, stimulated the production of the chemoattractant chemokines CXCL1 and CXCL2, and subsequent migration capacity of neutrophils in cultured wild-type (WT) and Tlr4(-/-) MCDs, but not in Tlr5(-/-) MCDs. UPEC can translocate across intact MCD layers without altering tight junctions. Strikingly, the invasion capacity and transcellular translocation of the UPEC strain HT7 were significantly lower in Tlr5(-/-) than in WT MCDs. The non-motile HT7ΔfliC mutant lacking flagellin also exhibited much lower translocation capacities than the HT7 isolates. Finally, Tlr5(-/-) kidneys exhibited less infiltrating neutrophils than WT kidneys one day after the transurethral inoculation of HT7, and greater delayed renal bacterial loads in the day 4 post-infected Tlr5(-/-) kidneys. Overall, these findings indicate that the epithelial TLR5 participates to renal antibacterial defence, but paradoxically favours the translocation of UPEC across intact MCD cell layers.

Calcineurin/NFAT signaling and innate host defence: a role for NOD1-mediated phagocytic functions

The calcineurin/nuclear factor of activated T cells (NFATs) signaling pathway plays a central role in T cell mediated adaptive immune responses, but a number of recent studies demonstrated that calcineurin/NFAT signaling also plays a key role in the control of the innate immune response by myeloid cells. Calcineurin inhibitors, such as cyclosporine A (CsA) and tacrolimus (FK506), are commonly used in organ transplantation to prevent graft rejection and in a variety of immune diseases. These immunosuppressive drugs have adverse effects and significantly increase host's susceptibility towards bacterial or fungal infections. Recent studies highlighted the role of NFAT signaling in fungal infection and in the control of the pattern recognition receptor nucleotide-binding oligomerization domain-containing protein 1 (NOD1), which predominantly senses invasive Gram-negative bacteria and mediates neutrophil phagocytic functions. This review summarises some of the current knowledge concerning the role of NFAT signaling in the innate immune response and the recent advances on NFAT-dependent inhibition of NOD1-mediated innate immune response caused by CsA, which may contribute to sensitizing transplant recipients to bacterial infection.

Cyclosporine A impairs nucleotide binding oligomerization domain (Nod1)-mediated innate antibacterial renal defenses in mice and human transplant recipients

Acute pyelonephritis (APN), which is mainly caused by uropathogenic Escherichia coli (UPEC), is the most common bacterial complication in renal transplant recipients receiving immunosuppressive treatment. However, it remains unclear how immunosuppressive drugs, such as the calcineurin inhibitor cyclosporine A (CsA), decrease renal resistance to UPEC. Here, we investigated the effects of CsA in host defense against UPEC in an experimental model of APN. We show that CsA-treated mice exhibit impaired production of the chemoattractant chemokines CXCL2 and CXCL1, decreased intrarenal recruitment of neutrophils, and greater susceptibility to UPEC than vehicle-treated mice. Strikingly, renal expression of Toll-like receptor 4 (Tlr4) and nucleotide-binding oligomerization domain 1 (Nod1), neutrophil migration capacity, and phagocytic killing of E. coli were significantly reduced in CsA-treated mice. CsA inhibited lipopolysaccharide (LPS)-induced, Tlr4-mediated production of CXCL2 by epithelial collecting duct cells. In addition, CsA markedly inhibited Nod1 expression in neutrophils, macrophages, and renal dendritic cells. CsA, acting through inhibition of the nuclear factor of activated T-cells (NFATs), also markedly downregulated Nod1 in neutrophils and macrophages. Silencing the NFATc1 isoform mRNA, similar to CsA, downregulated Nod1 expression in macrophages, and administration of the 11R-VIVIT peptide inhibitor of NFATs to mice also reduced neutrophil bacterial phagocytosis and renal resistance to UPEC. Conversely, synthetic Nod1 stimulating agonists given to CsA-treated mice significantly increased renal resistance to UPEC. Renal transplant recipients receiving CsA exhibited similar decrease in NOD1 expression and neutrophil phagocytosis of E. coli. The findings suggest that such mechanism of NFATc1-dependent inhibition of Nod1-mediated innate immune response together with the decrease in Tlr4-mediated production of chemoattractant chemokines caused by CsA may contribute to sensitizing kidney grafts to APN.

The CTX-M-15-producing Escherichia coli clone O25b: H4-ST131 has high intestine colonization and urinary tract infection abilities

Increasing numbers of pyelonephritis-associated uropathogenic Escherichia coli (UPEC) are exhibiting high resistance to antibiotic therapy. They include a particular clonal group, the CTX-M-15-producing O25b:H4-ST131 clone, which has been shown to have a high dissemination potential. Here we show that a representative isolate of this E. coli clone, referred to as TN03, has enhanced metabolic capacities, acts as a potent intestine- colonizing strain, and displays the typical features of UPEC strains. In a modified streptomycin-treated mouse model of intestinal colonization where streptomycin was stopped 5 days before inoculation, we show that TN03 outcompetes the commensal E. coli strains K-12 MG1655, IAI1, and ED1a at days 1 and 7. Using an experimental model of ascending UTI in C3H/HeN mice, we then show that TN03 colonized the urinary tract. One week after the transurethral inoculation of the TN03 isolates, the bacterial loads in the bladder and kidneys were significantly greater than those of two other UPEC strains (CFT073 and HT7) belonging to the same B2 phylogenetic group. The differences in bacterial loads did not seem to be directly linked to differences in the inflammatory response, since the intrarenal expression of chemokines and cytokines and the number of polymorphonuclear neutrophils attracted to the site of inflammation was the same in kidneys colonized by TN03, CFT073, or HT7. Lastly, we show that in vitro TN03 has a high maximum growth rate in both complex (Luria-Bertani and human urine) and minimum media. In conclusion, our findings indicate that TN03 is a potent UPEC strain that colonizes the intestinal tract and may persist in the kidneys of infected hosts.

A role for collecting duct epithelial cells in renal antibacterial defences

Urinary tract infections (UTIs), which are mainly due to uropathogenic Escherichia coli (UPEC), occur via the retrograde ascent of the bacteria along the urinary tract system. The adhesion and invasion mechanisms of UPEC have been extensively studied in bladder epithelial cells, but less is known about the role of renal tubule epithelial cells (RTEC) in renal antibacterial defences. This review considers recent advances in the understanding of the role of RTECs in inducing an innate immune response mediated by Toll-like receptors (TLRs) in experimental UTI. Collecting duct cells are a preferential site of adhesion of UPEC colonizing the kidneys. Epithelial TLR4 activation induces an inflammatory response and the recruitment of lipid rafts to the plasma membrane, both of which facilitate the transcytosis of non-cytolytic UPEC strains across intact collecting duct cell layers to invade the renal interstitium. Arginine vasopressin, which regulates water absorption in the collecting duct, also acts as a potent modulator of the TLR4-mediated intrarenal innate response caused by UPEC. The role of epithelial TLR5 in renal host defences is also discussed. These findings highlight the role of RTECs in triggering the innate immune response in the context of ascending UTIs.

Differential activation of Toll-like receptor-mediated apoptosis induced by hypoxia

Ischemia-reperfusion injury induces intense inflammatory response and tissue damages resulting from the capacity of endogenous constituents called damageassociated molecular patterns (DAMPs) released by damaged or necrotic cells, to activate signaling pathways mediated by receptors of the innate immune systems. Among them, two members of the Toll-like receptors (TLR) family, TLR2 and TLR4 have been shown to play key roles in the induction of inflammatory response and cell apoptosis in a variety of ischemic tissues. The oxidative stress injury caused by I/R injury has been attributed to the activation of MAP kinase pathways, including those of ERK, JNK and p38. Here, we summarise recent findings concerning the role of the protein phosphatase 5 involved in the selective regulation of TLR2-mediated ERK1/2 signaling and the identification of the key role of the non-phagocytic NADPH oxidase 4 producing reactive oxygen species in the control of TLR4-mediated apoptosis in murine models of renal I/R injury and in post-hypoxic kidney tubule cells. The identification of molecules signaling involved in the ER stress-induced apoptotic signaling cascade may therefore represent potential targets to prevent the induction of apoptosis in hypoxic tissues.

Heat shock protein gp96 and NAD(P)H oxidase 4 play key roles in Toll-like receptor 4-activated apoptosis during renal ischemia/reperfusion injury

Ischemia/reperfusion injury (IRI) causes inflammation and cell injury as a result of activating innate immune signaling. Toll-like receptor 4 (TLR4) has a key role in mediating kidney damages during IRI, but the downstream signaling pathway(s) stimulating apoptosis remains debated. In this study we show that TLR4 mediates MyD88-dependent activation of TNF receptor-associated factor 2, apoptosis signal-regulating kinase 1 (ASK1), and Jun N-terminal kinase (JNK) and p38 MAP kinases in ischemic-reperfused kidneys and posthypoxic renal tubule epithelial cells (RTECs). Hypoxia stimulated the expression of the endoplasmic-resident gp96, which co-immunoprecipitated TLR4, whereas silencing gp96 mRNA expression impaired hypoxia-induced apoptosis in TLR4-expressing RTECs. NAD(P)H oxidase 4 (NOX4) was shown to interact with TLR4 and to be required in lipopolysaccharide-induced production of reactive oxygen species (ROS). IRI stimulated the expression of a 28-kDa NOX4 spliced isoform abundantly expressed in wild-type RTECs, which co-immunoprecipitated with TLR4, but not with gp96 in TLR4-deficient RTECs. Silencing NOX4 mRNA expression impaired hypoxia-induced activation of ASK1 and both JNK and p38, leading to the inhibition of ROS production and apoptosis in posthypoxic TLR4-expressing RTECs. These findings show that, concomitantly to the activation of p38, the gp96/TLR4 interaction is required for activation of ASK1/JNK signaling in posthypoxic mouse RTECs, and that the 28-kDa NOX4 has a key role in TLR4-mediated apoptosis during renal IRI.

Clostridium septicum alpha-toxin forms pores and induces rapid cell necrosis

Alpha-toxin is the unique lethal virulent factor produced by Clostridium septicum, which causes traumatic or non-traumatic gas gangrene and necrotizing enterocolitis in humans. Here, we analyzed channel formation of the recombinant septicum alpha-toxin and characterized its activity on living cells. Recombinant septicum alpha-toxin induces the formation of ion-permeable channels with a single-channel conductance of about 175pS in 0.1M KCl in lipid bilayer membranes, which is typical for a large diffusion pore. Septicum alpha-toxin channels remained mostly in the open configuration, displayed no lipid specificity, and exhibited slight anion selectivity. Septicum alpha-toxin caused a rapid decrease in the transepithelial electrical resistance of MDCK cell monolayers grown on filters, and induced a rapid cell necrosis in a variety of cell lines, characterized by cell permeabilization to propidium iodide without DNA fragmentation and activation of caspase-3. Septicum alpha-toxin also induced a rapid K(+) efflux and ATP depletion. Incubation of the cells in K(+)-enriched medium delayed cell death caused by septicum alpha-toxin or epsilon-toxin, another potent pore-forming toxin, suggesting that the rapid loss of intracellular K(+) represents an early signal of pore-forming toxins-mediated cell necrosis.

Heat shock protein gp96 interacts with protein phosphatase 5 and controls toll-like receptor 2 (TLR2)-mediated activation of extracellular signal-regulated kinase (ERK) 1/2 in post-hypoxic kidney cells

Ischemia/reperfusion injury (IRI) induces an innate immune response, leading to an inflammatory reaction and tissue damage that have been attributed to engagement of the Toll-like receptor (TLR) 2 and 4. However, the respective roles of TLR2 and/or TLR4 in mediating downstream activation of mitogen-activated protein kinase (MAPK) pathways during IRI have not been fully elucidated. Here we show that extracellular signal-regulated kinase (ERK)1/2 is activated in both intact kidneys and cultured renal tubule epithelial cells (RTECs) from wildtype and Tlr4 knockout mice, but not those from Tlr2 knockout mice subjected to transient ischemia. Geldanamycin (GA), an inhibitor of heat shock protein 90 and reticulum endoplasmic-resident gp96, and gp96 mRNA silencing (siRNA), did not affect ERK1/2 activation in either post-hypoxic wild-type or Tlr4-deficient RTECs, but did restore its activation in post-hypoxic Tlr2-deficient RTECs. Immunoprecipitation studies revealed that gp96 co-immunoprecipitates with the serine-threonine protein phosphatase 5 (PP5), identified as a negative modulator of the mitogen extracellular kinase (MEK)-ERK pathway, in unstressed wild-type and post-hypoxic Tlr2-deficient RTECs. In contrast, PP5 co-immunoprecipitation with gp96 was strikingly reduced in post-hypoxic wild-type RTECs, suggesting that the inactivation of PP5 resulting from the dissociation of PP5 from gp96 allows the activation of ERK1/2 to occur. Inhibition of PP5 by okadaic acid, and Pp5 siRNA also restored TLR2-mediated phosphorylation of ERK1/2, and apoptosis signal-regulating kinase 1 (ASK1)/c-Jun N-terminal kinase (JNK)-mediated apoptosis in post-hypoxic Tlr2-deficient RTECs. These findings indicate that gp96 interacts with PP5 and controls TLR2-mediated induction of ERK1/2 in post-hypoxic renal tubule cells.

TLR4 facilitates translocation of bacteria across renal collecting duct cells

Uropathogenic Escherichia coli (UPEC) are the most frequent causes of urinary tract infections and pyelonephritis. Renal medullary collecting duct (MCD) cells are the intrarenal site to which UPEC strains prefer to adhere and initiate an inflammatory response, but the ability of UPEC strains to translocate across impermeant MCD cells has not been demonstrated definitively. Here, several UPEC strains adhered to the apical surface and translocated across confluent murine inner MCD cells grown on filters. UPEC strains expressing cytolytic and vacuolating cytotoxins disrupted the integrity of cell layers, whereas noncytolytic UPEC strains passed through the cell layers without altering tight junctions. Apical-to-basal transcellular translocation was dramatically reduced after extinction of Toll-like receptor 4 (TLR4) and the lipid raft marker caveolin-1 by small interfering RNA. Furthermore, disruption of lipid raft integrity by filipin III and methyl-beta-cyclodextrin significantly reduced both the transcellular translocation of UPEC across murine inner MCD cell layers and the stimulation of proinflammatory mediators. Bacterial translocation was also significantly reduced in primary cultures of TLR4-deficient mouse MCD cells compared with MCD cells from wild-type mice. Benzyl alcohol, an anesthetic that enhances membrane fluidity, favored the recruitment of caveolin-1 in lipid rafts and increased the translocation of UPEC across cultured TLR4-deficient MCD cells. These findings demonstrate that the transcellular translocation of UPEC strains across impermeant layers of MCD cells may occur through lipid rafts via a TLR4-facilitated process.

Kir4.1/Kir5.1 channel forms the major K+ channel in the basolateral membrane of mouse renal collecting duct principal cells

K(+) channels in the basolateral membrane of mouse cortical collecting duct (CCD) principal cells were identified with patch-clamp technique, real-time PCR, and immunohistochemistry. In cell-attached membrane patches, three K(+) channels with conductances of approximately 75, 40, and 20 pS were observed, but the K(+) channel with the intermediate conductance (40 pS) predominated. In inside-out membrane patches exposed to an Mg(2+)-free medium, the current-voltage relationship of the intermediate-conductance channel was linear with a conductance of 38 pS. Addition of 1.3 mM internal Mg(2+) had no influence on the inward conductance (G(in) = 35 pS) but reduced outward conductance (G(out)) to 13 pS, yielding a G(in)/G(out) of 3.2. The polycation spermine (6 x 10(-7) M) reduced its activity on inside-out membrane patches by 50% at a clamp potential of 60 mV. Channel activity was also dependent on intracellular pH (pH(i)): a sigmoid relationship between pH(i) and channel normalized current (NP(o)) was observed with a pK of 7.24 and a Hill coefficient of 1.7. By real-time PCR on CCD extracts, inwardly rectifying K(+) (Kir)4.1 and Kir5.1, but not Kir4.2, mRNAs were detected. Kir4.1 and Kir5.1 proteins cellularly colocalized with aquaporin 2 (AQP2), a specific marker of CCD principal cells, while AQP2-negative cells (i.e., intercalated cells) showed no staining. Dietary K(+) had no influence on the properties of the intermediate-conductance channel, but a Na(+)-depleted diet increased its open probability by approximately 25%. We conclude that the Kir4.1/Kir5.1 channel is a major component of the K(+) conductance in the basolateral membrane of mouse CCD principal cells.

Hormonal control of the renal immune response and antibacterial host defense by arginine vasopressin

Ascending urinary tract infection (UTI) and pyelonephritis caused by uropathogenic Escherichia coli (UPEC) are very common infections that can cause severe kidney damage. Collecting duct cells, the site of hormonally regulated ion transport and water absorption controlled by vasopressin, are the preferential intrarenal site of bacterial adhesion and initiation of inflammatory response. We investigated the effect of the potent V2 receptor (V2R) agonist deamino-8-D-arginine vasopressin (dDAVP) on the activation of the innate immune response using established and primary cultured collecting duct cells and an experimental model of ascending UTI. dDAVP inhibited Toll-like receptor 4–mediated nuclear factor κB activation and chemokine secretion in a V2R-specific manner. The dDAVP-mediated suppression involved activation of protein phosphatase 2A and required an intact cystic fibrosis transmembrane conductance regulator Cl⁻ channel. In vivo infusion of dDAVP induced a marked fall in proinflammatory mediators and neutrophil recruitment, and a dramatic rise in the renal bacterial burden in mice inoculated with UPECs. Conversely, administration of the V2R antagonist SR121463B to UPEC-infected mice stimulated both the local innate response and the antibacterial host defense. These findings evidenced a novel hormonal regulation of innate immune cellular activation and demonstrate that dDAVP is a potent modulator of microbial-induced inflammation in the kidney.

Pore-forming epsilon toxin causes membrane permeabilization and rapid ATP depletion-mediated cell death in renal collecting duct cells

Clostridium perfringens epsilon toxin (ET) is a potent pore-forming cytotoxin causing fatal enterotoxemia in livestock. ET accumulates in brain and kidney, particularly in the renal distal-collecting ducts. ET binds and oligomerizes in detergent-resistant membranes (DRMs) microdomains and causes cell death. However, the causal linkage between membrane permeabilization and cell death is not clear. Here, we show that ET binds and forms 220-kDa insoluble complexes in plasma membrane DRMs of renal mpkCCD(cl4) collecting duct cells. Phosphatidylinositol-specific phospholipase C did not impair binding or the formation of ET complexes, suggesting that the receptor for ET is not GPI anchored. ET induced a dose-dependent fall in the transepithelial resistance and potential in confluent cells grown on filters, transiently stimulated Na+ absorption, and induced an inward ionic current and a sustained rise in [Ca2+]i. ET also induced rapid depletion of cellular ATP, and stimulated the AMP-activated protein kinase, a metabolic-sensing Ser/Thr kinase. ET also induced mitochondrial membrane permeabilization and mitochondrial-nuclear translocation of apoptosis-inducing factor, a potent caspase-independent cell death effector. Finally, ET induced cell necrosis characterized by a marked reduction in nucleus size without DNA fragmentation. DRM disruption by methyl-beta-cyclodextrin impaired ET oligomerization, and significantly reduced the influx of Na+ and [Ca2+]i, but did not impair ATP depletion and cell death caused by the toxin. These findings indicate that ET causes rapid necrosis of renal collecting duct cells and establish that ATP depletion-mediated cell death is not strictly correlated with the plasma membrane permeabilization and ion diffusion caused by the toxin.

Transimmortalized proximal tubule and collecting duct cell lines derived from the kidneys of transgenic mice

This review summarizes the strategy of cellular immortalization based on the principle of targeted oncogenesis in transgenic mice, used to establish models of transimmortalized renal proximal tubule cells, referred to as PKSV-PCT and PKSV-PR-cells, and collecting duct principal cells, referred to as mpkCCD(cl4) cells. These cell lines have maintained for long-term passages the main biochemical and functional properties of the parental cells from which they were derived. Proximal tubule PKSV-PCT and PKSV-PR cells have been proved to be suitable cell systems for toxicological and pharmacological studies. They also permitted the establishment of a model of multidrug-resistant (MDR) renal epithelial tubule cells, PKSV-PR(col50), which have served for the study of both MDR-dependent extrusion of chemotherapeutic drugs and inappropriate accumulation of weak base anthracyclines in intracellular acidic organelles. The novel collecting duct cell line mpkCCD(cl4), which has maintained the characteristics of tight epithelial cells, in particular Na(+) absorption stimulated by aldosterone, has been extensively used for pharmacological studies related to the regulation of ion transport. These cells have permitted the identification of several aldosterone-induced proteins playing a key role in the regulation of Na(+) absorption mediated by the epithelial Na(+) channel ENaC. Recent studies have also provided evidence that these cell lines represent valuable cell systems for the study of host-pathogen interactions and the analysis of the role of renal tubule epithelial cells in the induction of inflammatory response caused by uropathogens that may lead to severe renal damage.

The synthetic androgen methyltrienolone (r1881) acts as a potent antagonist of the mineralocorticoid receptor

Aldosterone binds to the mineralocorticoid receptor (MR) and exerts fine control over Na+ absorption in renal collecting duct cells (CCDs). Many natural and synthetic steroids can also bind to the MR to produce agonist or antagonist effects. Here, we investigate whether androgenic hormones act as MR agonist or antagonist ligands in CCDs. Testosterone (T), dihydrotestosterone (DHT), and methyltrienolone (R1881), a synthetic androgen agonist, all bind to the MR. R1881 displayed the same affinity for MR as aldosterone. Androgens did not activate the MR transiently expressed in human embryonic kidney 293T cells but did antagonize aldosterone-induced MR trans-activation activity (R1881>DHT>T). Short-circuit current (Isc) experiments, used to measure transepithelial Na+ transport, revealed that 10(-5) M T and DHT or R1881 prevented the increase in the amiloride-sensitive component of Isc caused by aldosterone in mouse mpkCCDcl4 collecting duct cells partially and totally, respectively. In contrast, androgens had no effect on stimulated Isc elicited by the specific glucocorticoid agonist 11beta,17beta-dihydroxy-17alpha-(1-propynyl) and rost-1,4,6-trien-3-one (RU26988). Docking of steroids within the crystal structure of the ligand-binding domain of MR, together with trans-activation studies, revealed that the contacts between the 17beta-hydroxyl group of androgens and the Asn770, Cys942, and Thr945 residues of the ligand-binding cavity stabilize ligand binding complexes but are not strong enough to keep the receptor in its active state. Altogether, these findings indicate that androgen ligands, particularly R1881, act as MR antagonists in aldosterone target cells and provide new insights into the requirements for MR activation to occur and for the designing of new selective MR antagonists.

Renal collecting duct epithelial cells react to pyelonephritis-associated Escherichia coli by activating distinct TLR4-dependent and -independent inflammatory pathways

TLR4 plays a central role in resistance to pyelonephritis caused by uropathogenic Escherichia coli (UPEC). It has been suggested that renal tubule epithelial cells expressing TLRs may play a key role in inflammatory disorders and in initiating host defenses. In this study we used an experimental mouse model of ascending urinary tract infection to show that UPEC isolates preferentially adhered to the apical surface of medullary collecting duct (MCD) intercalated cells. UPEC-infected C3H/HeJ (Lps(d)) mice carrying an inactivating mutation of tlr4 failed to clear renal bacteria and exhibited a dramatic slump in proinflammatory mediators as compared with infected wild-type C3H/HeOuJ (Lps(n)) mice. However, the level of expression of the leukocyte chemoattractants MIP-2 and TNF-alpha still remained greater in UPEC-infected than in naive C3H/HeJ (Lps(d)) mice. Using primary cultures of microdissected Lps(n) MCDs that expressed TLR4 and its accessory molecules MD2, MyD88, and CD14, we also show that UPECs stimulated both a TLR4-mediated, MyD88-dependent, TIR domain-containing adaptor-inducing IFN-beta-independent pathway and a TLR4-independent pathway, leading to bipolarized secretion of MIP-2. Stimulation by UPECs of the TLR4-mediated pathway in Lps(n) MCDs leads to the activation of NF-kappaB, and MAPK p38, ERK1/2, and JNK. In addition, UPECs stimulated TLR4-independent signaling by activating a TNF receptor-associated factor 2-apoptosis signal-regulatory kinase 1-JNK pathway. These findings demonstrate that epithelial collecting duct cells are actively involved in the initiation of an immune response via several distinct signaling pathways and suggest that intercalated cells play an active role in the recognition of UPECs colonizing the kidneys.

Regulation of NaCl transport in the renal collecting duct: lessons from cultured cells

The fine control of NaCl absorption regulated by hormones takes place in the distal nephron of the kidney. In collecting duct principal cells, the epithelial sodium channel (ENaC) mediates the apical entry of Na(+), which is extruded by the basolateral Na(+),K(+)-ATPase. Simian virus 40-transformed and "transimmortalized" collecting duct cell lines, derived from transgenic mice carrying a constitutive, conditionally, or tissue-specific promoter-regulated large T antigen, have been proven to be valuable tools for studying the mechanisms controlling the cell surface expression and trafficking of ENaC and Na(+),K(+)-ATPase. These cell lines have made it possible to identify sets of aldosterone- and vasopressin-stimulated proteins, and have provided new insights into the concerted mechanism of action of serum- and glucocorticoid-inducible kinase 1 (Sgk1), ubiquitin ligase Nedd4-2 (neural precursor cell-expressed, developmentally down-regulated protein 4-2), and 14-3-3 regulatory proteins in modulating ENaC-mediated Na(+) currents. Epidermal growth factor and induced leucine zipper protein have also been shown to repress and stimulate ENaC-dependent Na(+) absorption, respectively, by activating or repressing the mitogen-activated protein kinase externally regulated kinase(1/2). Overall, these findings have provided evidence suggesting that multiple pathways are involved in regulating NaCl absorption in the distal nephron.

Interleukin-8-induced priming of neutrophil oxidative burst requires sequential recruitment of NADPH oxidase components into lipid rafts

The superoxide-producing phagocyte NADPH oxidase consists of a membrane-bound flavocytochrome b(558), the cytosol factors p47(phox), p67(phox), p40(phox), and the small GTPase Rac2, which translocate to the membrane to assemble the active complex following neutrophil activation. Interleukin-8 (IL-8) does not activate NADPH oxidase, but potentiates the oxidative burst induced by stimuli such as formyl-methionyl-leucyl-phenylalanine (fMLP) via a priming mechanism. The effect of IL-8 on the components of NADPH oxidase during the priming process has never been investigated in human neutrophils. Here we showed that within 3 min, IL-8 treatment enhanced the Btk- and ERK1/2-dependent phosphorylation of p47(phox), as well as the recruitment of flavocytochrome b(558), p47(phox), and Rac2 into cholesterol-enriched detergent-resistant microdomains (or lipid rafts). Conversely, IL-8 treatment lasting 15 min failed to recruit flavocytochrome b(558), p47(phox), or Rac2, but did enhance the Btk- and p38 MAPK-dependent phosphorylation and the translocation of p67(phox) into detergent-resistant microdomains. Moreover, methyl-beta-cyclodextrin, which disrupts lipid rafts, inhibited IL-8-induced priming in response to fMLP. Our findings indicate that IL-8-induced priming of the oxidative burst in response to fMLP involves a sequential assembly of the NADPH oxidase components in the lipid rafts of neutrophils.

NAD(P)H oxidase Nox-4 mediates 7-ketocholesterol-induced endoplasmic reticulum stress and apoptosis in human aortic smooth muscle cells

The mechanisms involved in the cytotoxic action of oxysterols in the pathogenesis of atherosclerosis still remain poorly understood. Among the major oxysterols present in oxidized low-density lipoprotein, we show here that 7-ketocholesterol (7-Kchol) induces oxidative stress and/or apoptotic events in human aortic smooth muscle cells (SMCs). This specific effect of 7-Kchol is mediated by a robust upregulation (threefold from the basal level) of Nox-4, a reactive oxygen species (ROS)-generating NAD(P)H oxidase homologue. This effect was highlighted by silencing Nox-4 expression with a specific small interfering RNA, which significantly reduced the 7-Kchol-induced production of ROS and abolished apoptotic events. Furthermore, the 7-Kchol activating pathway included an early triggering of endoplasmic reticulum stress, as assessed by transient intracellular Ca(2+) oscillations, and the induction of the expression of the cell death effector CHOP and of GRP78/Bip chaperone via the activation of IRE-1, all hallmarks of the unfolded protein response (UPR). We also showed that 7-Kchol activated the IRE-1/Jun-NH(2)-terminal kinase (JNK)/AP-1 signaling pathway to promote Nox-4 expression. Silencing of IRE-1 and JNK inhibition downregulated Nox-4 expression and subsequently prevented the UPR-dependent cell death induced by 7-Kchol. These findings demonstrate that Nox-4 plays a key role in 7-Kchol-induced SMC death, which is consistent with the hypothesis that Nox-4/oxysterols are involved in the pathogenesis of atherosclerosis.

Vasopressin-stimulated CFTR Cl- currents are increased in the renal collecting duct cells of a mouse model of Liddle's syndrome

Liddle's syndrome is a genetic form of hypertension linked to Na(+) retention caused by activating mutations in the COOH terminus of the beta or gamma subunit of the epithelial sodium channel (ENaC). In this study, we used the short-circuit current (I(sc)) method to investigate the effects of deamino-8-d-arginine vasopressin (dDAVP) on Na(+) and Cl(-) fluxes in primary cultures of cortical collecting ducts (CCDs) microdissected from the kidneys of mice with Liddle's syndrome carrying a stop codon mutation, corresponding to the beta-ENaC R(566) stop mutation (L) found in the original pedigree. Compared to wild-type (+/+) CCD cells, untreated L/+ and L/L CCD cells exhibited 2.7- and 4.2-fold increases, respectively, in amiloride-sensitive (Ams) I(sc), reflecting ENaC-dependent Na(+) absorption. Short-term incubation with dDAVP caused a rapid and significant increase (approximately 2-fold) in Ams I(sc) in +/+, but not in L/+ or L/L CCD cells. In sharp contrast, dDAVP induced a greater increase in 5-nitro-2-(3-phenylpropamino)benzoate (NPPB)-inhibited apical Cl(-) currents in amiloride-treated L/L and L/+ cells than in their +/+ counterparts. I(sc) recordings performed under apical ion substituted conditions revealed that the dDAVP-stimulated apical secretion of Cl(-), which was absent in cultured CCDs lacking CFTR, was 1.8-fold greater in L/+ and 3.7-fold greater in L/L CCD cells than in their +/+ CCD counterparts. After the basal membrane had been permeabilized with nystatin and a basal-to-apical Cl(-) gradient had been imposed, dDAVP also stimulated larger Cl(-) currents across L/L and L/+ CCD layers than +/+ CCD layers. These findings demonstrate that vasopressin stimulates greater apical CFTR Cl(-) conductance in the renal CCD cells of mice with Liddle's syndrome than in wild-type mice. This effect could contribute to the enhanced NaCl reabsorption observed in the distal nephron of patients with Liddle's syndrome.

Extracellular Hypotonicity Increases Na,K-ATPase Cell Surface Expression via Enhanced Na+ Influx in Cultured Renal Collecting Duct Cells

In the renal collecting duct (CD), the Na,K-ATPase, which provides the driving force for Na+ absorption, is under tight multifactorial control. Because CD cells are physiologically exposed to variations of interstitial and tubular fluid osmolarities, the effects of extracellular anisotonicity on Na,K-ATPase cell surface expression were studied. Results show that hypotonic conditions increased, whereas hypertonic conditions had no effect on Na,K-ATPase cell surface expression in confluent mpkCCDcl4 cells. Incubating cells with amphotericin B, which increases [Na+]i, under isotonic or anisotonic conditions, revealed that Na,K-ATPase recruitment to the cell surface was not directly related to variations of cell volume and osmolarity. The effects of amphotericin B and extracellular hypotonicity were not additive, and both were prevented by protein kinase A and proteasome inhibitors, suggesting a common mechanism of action. In line with this hypothesis, extracellular hypotonicity induced a sustained stimulation of the amiloride-sensitive short-circuit current, indicating increased Na+ influx through the apical epithelial Na+ channel. Moreover, inhibiting apical Na+ entry by amiloride, a blocker of epithelial Na+ channel, or incubating cells in Na+ -free medium prevented the cell surface recruitment of Na,K-ATPase in response to extracellular hypotonicity. Altogether, these findings strongly suggest that extracellular hypotonicity stimulates apical Na+ influx leading to increased [Na+]i, protein kinase A activation, and recruitment of Na,K-ATPase units to the cell surface of mpkCCDcl4 cells.

Characterization of a murine renal distal convoluted tubule cell line for the study of transcellular calcium transport

To unravel the molecular regulation of renal transcellular Ca(2+) transport, a murine distal convoluted tubule (mpkDCT) cell line derived from distal convoluted tubules (DCT) microdissected from a SV-PK/Tag transgenic mouse was characterized. This cell line originated from DCT only, as mRNA encoding for the DCT marker thiazide-sensitive Na(+)/Cl(-) cotransporter was expressed, whereas mRNA encoding for the connecting tubule and collecting duct marker aquaporin-2 was not detected, as determined by reverse-transcriptase PCR. mpkDCT cells expressed mRNA encoding the Ca(2+) channels TRPV5 and TRPV6 and other key players necessary for transcellular Ca(2+) transport, i.e., calbindin-D(9k), calbindin-D(28k), plasma membrane Ca(2+)-ATPase isoform 1b, and Na(+)/Ca(2+) exchanger 1. Primary cultures of DCT cells exhibited net transcellular Ca(2+) transport of 0.4 +/- 0.1 nmol.h(-1).cm(-2), whereas net transcellular Ca(2+) transport across mpkDCT cells was significantly higher at 2.4 +/- 0.4 nmol.h(-1).cm(-2). Transcellular Ca(2+) transport across mpkDCT cells was completely inhibited by ruthenium red, an inhibitor of TRPV5 and TRPV6, but not by the voltage-operated Ca(2+) channel inhibitors felodipine and verapamil. With the use of patch-clamp analysis, the IC(50) of ruthenium red on Na(+) currents was between the values measured for TRPV5- and TRPV6-expressing HEK 293 cells, suggesting that TRPV5 and/or TRPV6 is possibly active in mpkDCT cells. Forskolin in combination with IBMX, 1,25-dihydroxyvitamin D(3), and 1-deamino-8-d-arginine vasopressin increased transcellular Ca(2+) transport, whereas PMA and parathyroid hormone had no significant effect. In conclusion, the murine mpkDCT cell line provides a unique cell model in which to study the molecular regulation of transcellular Ca(2+) transport in the kidney in vitro.

Dysfunction of the epithelial sodium channel expressed in the kidney of a mouse model for Liddle syndrome

The Liddle syndrome is a dominant form of salt-sensitive hypertension resulting from mutations in the beta or gamma subunit of ENaC. A previous study established a mouse model carrying a premature Stop codon corresponding to the R(566stop) mutation (L) found in the original pedigree that recapitulates to a large extent the human disease. This study investigated the renal Na(+) transport in vivo, ex vivo (intact perfused tubules), and in vitro (primary cultured cortical collecting ducts [CCD]). In vivo, upon 6 to 12 h of salt repletion, after 1 week of low-salt diet, the L/L mice showed a delayed urinary sodium excretion, despite a lower aldosterone secretion as compared with controls. After 6 h salt of repletion, ENaC gamma subunit is rapidly removed from the apical plasma membrane in wild-type mice, whereas it is retained at the apical membrane in L/L mice. Ex vivo, isolated perfused CCD from L/L mice exhibited higher transepithelial potential differences than perfused CCD isolated from +/+ mice. In vitro, confluent primary cultures of CCD microdissected from L/L kidneys grown on permeable filters exhibited significant lower transepithelial electrical resistance and higher negative potential differences than their cultured L/+ and +/+ CCD counterparts. The equivalent short-circuit current (I(eq)) and the amiloride-sensitive I(eq) was approximately twofold higher in cultured L/L CCD than in +/+ CCD. Aldosterone (5 x 10(-7)M for 3 h) further increased I(eq) from cultured L/L CCD. Thus, this study brings three independent lines of evidence for the constitutive hyperactivity of ENaC in CCD from mice harboring the Liddle mutation.

Cyclosporin increases the density of angiotensin II subtype 1 (AT1) receptors in mouse medullary thick ascending limb cells

BACKGROUND

Cyclosporin A (CsA), a potent immunosuppressive agent, can be nephrotoxic. Because clinical studies have suggested that the intrarenal renin-angiotensin system may be involved in the mechanism responsible for CsA nephrotoxicity, we have analysed the effects of CsA on angiotensin II (Ang II) receptors in medullary thick ascending limb (mTAL) cells known to be sensitive to the action of CsA.

METHODS

Experiments were carried out on subcultured mouse mTAL cells. The expression of mRNA of Ang II subtype 1 and 2 (AT(1) and AT(2)) receptors was investigated using reverse transcription-polymerase chain reaction (RT-PCR). [(3)H]Ang II was used for radioligand and binding studies. Fluorimetric recordings using the fluorescent dye fura-2/AM were performed to determine the effect of CsA on the intracellular calcium ([Ca(2+)]i) content of untreated and Ang II-treated mTAL cells.

RESULTS

Subcultured mTAL cells expressed AT(1) and AT(2) Ang II receptor mRNAs, and binding studies revealed that the AT(1) receptors were the predominant Ang II receptor subtype ( approximately 90%) in mTAL cells. CsA (100 ng/ml, 24 h) increased (1.7-fold) the number of Ang II receptors (untreated, 315.8; +CsA, 543.6 fmol/mg protein) without altering the K(D) (untreated, 7.16; +CsA, 7.06 nM). CsA also significantly increased the level of [Ca(2+)]i measured in cultured mTAL cells both in the basal state (-CSA, 72.2; +CsA, 93.4 nM/10(6) cells) and in the presence of Ang II (-CSA, 97.8; +CsA, 206.3 nM/10(6) cells).

CONCLUSIONS

These findings suggest that the increase in Ang II AT(1) receptors and [Ca(2+)]i caused by CsA may be involved in the mechanism(s) responsible for CsA nephrotoxicity.

Clostridium perfringens epsilon toxin rapidly decreases membrane barrier permeability of polarized MDCK cells

Epsilon toxin is produced by Clostridium perfringens types B and D which are responsible for fatal intestinal diseases in animals. The main biological activity of epsilon toxin is the production of oedema in various organs. We have previously found that epsilon toxin forms a large membrane complex in MDCK cells which is not internalized into cell, and induces cell volume enlargement and loss of cell viability (Petit, L., Gibert, M., Gillet, D., Laurent-Winter, C., Boquet, P., Popoff, M. R. (1997) J Bacteriol 179, 6480-6487). Here, we show that epsilon toxin is very potent to decrease the trans-epithelial electrical resistance of polarized MDCK cells grown on filters without altering the organization of the junctional complexes. The dose-dependent decrease in trans-epithelial electrical resistance, more marked when the toxin was applied to the apical side than to the basal side of MDCK cells, was associated with a moderate increase of the paracellular permeability to low-molecular-weight compounds but not to macromolecules. Epsilon toxin probably acts by forming large membrane pores which permit the flux of ions and other molecules such as the entry of propidium iodide and finally to the loss of cell viability.

Inhibitors of vacuolar H+-ATPase impair the preferential accumulation of daunomycin in lysosomes and reverse the resistance to anthracyclines in drug-resistant renal epithelial cells

It has been suggested that the inappropriate sequestration of weak-base chemotherapeutic drugs in acidic vesicles by multidrug-resistance (MDR) cells contributes to the mechanisms of drug resistance. The function of the acidic lysosomes can be altered in MDR cells, and so we investigated the effects of lysosomotropic agents on the secretion of lysosomal enzymes and on the intracellular distribution of the weak-base anthracycline daunomycin in drug-resistant renal proximal tubule PKSV-PR(col50) cells and their drug-sensitive PKSV-PR cell counterparts. Imaging studies using pH-dependent lysosomotropic dyes revealed that drug-sensitive and drug-resistant cells exhibited a similar acidic lysosomal pH (around 5.6-5.7), but that PKSV-PR(col50) cells contained more acidic lysosomes and secreted more of the lysosomal enzymes N -acetyl-beta-hexosaminidase and beta-glucuronidase than their parent PKSV-PR cells. Concanamycin A (CCM A), a potent inhibitor of the vacuolar H(+)-ATPase, but not the P-glycoprotein modulator verapamil, stimulated the secretion of N -acetyl-beta-hexosaminidase in both drug-sensitive and drug-resistant cells. Fluorescent studies and Percoll density gradient fractionation studies revealed that daunomycin accumulated predominantly in the lysosomes of PKSV-PR(col50) cells, whereas in PKSV-PR cells the drug was distributed evenly throughout the nucleo-cytoplasmic compartments. CCM A did not impair the cellular efflux of daunomycin, but induced the rapid nucleo-cytoplasmic redistribution of the drug in PKSV-PR(col50) cells. In addition, CCM A and bafilomycin A1 almost completely restored the sensitivity of these drug-resistant cells to daunomycin, doxorubicin and epirubicin. These findings indicate that lysosomotropic agents that impair the acidic-pH-dependent accumulation of weak-base chemotherapeutic drugs may reverse anthracycline resistance in MDR cells with an expanded acidic lysosomal compartment.

11beta-hydroxyprogesterone acts as a mineralocorticoid agonist in stimulating Na+ absorption in mammalian principal cortical collecting duct cells

The binding of mineralocorticoid hormones to the mineralocorticoid receptor is the first step in a cascade of events leading to the stimulation of Na(+) reabsorption by renal cortical collecting duct (CCD) principal cells. The agonist properties of mineralocorticoid hormones are linked to contacts between their 21-hydroxyl group and Asn770, a residue of the ligand-binding domain of the human mineralocorticoid receptor (hMR). Here, we investigate whether the presence of a hydroxyl group at position 11, 17, or 20 could also alter the activity of progesterone (P), a mineralocorticoid antagonist without the 21-hydroxyl group. Both 17 alpha-hydroxyprogesterone (17OHP) and 20 alpha-hydroxyprogesterone (20OHP) antagonized the aldosterone-induced trans-activation activity (IC(50): 17OHP, 10(-7) M; 20OHP, 10(-8) M) of the hMR transiently expressed in COS-7 cells lacking steroid receptors. In cultured mouse mpkCCD(cl4) principal cells, 17OHP and 20OHP also prevented the aldosterone-stimulated amiloride-sensitive component of the short-circuit current (Ams I(sc)), reflecting Na(+) absorption mediated by the epithelial Na(+) channel (ENaC). In contrast, 11 beta-hydroxyprogesterone (11OHP) activated the transiently expressed hMR in COS-7 cells in a dose-dependent manner (ED(50): 10(-8) M) and, like aldosterone, stimulated Ams I(sc) in mpkCCD(cl4) cells. Docking 11OHP within the hMR-ligand-binding domain homology model revealed that the agonist activity of 11OHP is caused by contacts between its 11 beta-hydroxyl group and Asn770. Furthermore, 11OHP was unable to activate the mutant hMR/N770A, in which Ala is substituted for Asn at position 770. These findings demonstrate that in the absence of the 21-hydroxyl group, the 11 beta-hydroxyl group can produce the contact with the hMR-Asn770 required for the hMR activation leading to stimulated Na(+) absorption.

Lymphoepithelial interactions trigger specific regulation of gene expression in the M cell-containing follicle-associated epithelium of Peyer's patches

In the intestine, the follicle-associated epithelium (FAE) of Peyer's patches (PP) performs Ag sampling as the first step in developing immune responses. Depending on the species, this epithelium contains 10-50% of M cells, which act as regulated gates in epithelial barriers that can be used opportunistically by pathogens to invade their host. However, the mechanisms involved in the differentiation and uptake processes of M cells are not known, in part because their limited number in the intestinal mucosa has hampered molecular and biochemical studies. In this work we provide evidence that PP lymphocytes can themselves modulate gene expression in PP in vivo and in an in vitro model of FAE. Transgenic mice carrying a reporter gene under the control of a modified L-pyruvate kinase promoter (SVPK) exhibit strong transgene expression in PP and FAE, but not in the adjacent villous cells. We used the mouse intestinal epithelial cell line m-IC(cl2) transfected with the SVPK promoter fused to beta-galactosidase to investigate the direct effect of PP lymphocytes on SVPK promoter activity. beta-Galactosidase expression was 4.4-fold higher in transfected m-IC(cl2) cells when they were cultured with PP lymphocytes. Conversely, green fluorescent protein expression was 1.8-fold lower in stably transfected differentiated intestinal Caco-2(cl1) cells with the sucrase isomaltase promoter fused to green fluorescent protein cDNA when they were cultured with PP lymphocytes, indicating that the in vivo FAE down-regulation of sucrase isomaltase promoter is transcriptionally regulated.

Long term regulation of aquaporin-2 expression in vasopressin-responsive renal collecting duct principal cells

Fine regulation of water reabsorption by the antidiuretic hormone [8-arginine]vasopressin (AVP) occurs in principal cells of the collecting duct and is largely dependent on regulation of the aquaporin-2 (AQP2) water channel. AVP-inducible long term AQP2 expression was investigated in immortalized mouse cortical collecting duct principal cells. Combined RNase protection assay, Western blot, and immunofluorescence analyses revealed that physiological concentrations of AVP added to the basal side, but not to the apical side, of cells grown on filters induced both AQP2 mRNA and apical protein expression. The stimulatory effect of AVP on AQP2 expression followed a V(2) receptor-dependent pathway because [deamino-8-d-arginine]vasopressin (dDAVP), a specific V(2) receptor agonist, produced the same effect as AVP, whereas the V(2) antagonist SR121463B antagonized action of both AVP and dDAVP. Moreover, forskolin and cyclic 8-bromo-AMP fully reproduced the effects of AVP on AQP2 expression. Analysis of protein degradation pathways showed that inhibition of proteasomal activity prevented synthesis of AVP-inducible AQP2 mRNA and protein. Once synthesized, AQP2 protein was quickly degraded, a process that involves both the proteasomal and lysosomal pathways. This is the first study that delineates induction and degradation mechanisms of AQP2 endogenously expressed by a renal collecting duct principal cell line.

An inward rectifier K(+) channel at the basolateral membrane of the mouse distal convoluted tubule: similarities with Kir4-Kir5.1 heteromeric channels

In this study, K(+) channels present in the basolateral membrane of the distal convoluted tubule (DCT) were investigated using patch-clamp methods. In addition, Kir4.1, Kir4.2 and Kir5.1 inward rectifier channels were investigated using RT-PCR and immunohistochemistry (Kir4.1). DCTs were microdissected from collagenase-treated mouse kidneys. One type of K(+) channel was detected in about 50 % of cell-attached patches from the DCT basolateral membrane; this channel was inwardly rectifying and had an inward conductance (g(in)) of approximately 40 pS at an external [K(+)] of 145 mM. The current-voltage relationship was linear when inside-out patches were exposed to a Mg(2+)-free medium. Mg(2+) at a concentration of 1.2 mM considerably reduced the outward conductance (g(out)), yielding a g(in)/g(out) ratio of approximately 4.7. The polycation spermine (5 x 10(-7) M) reduced the open probability (P(o)) by 50 %. Channel activity was dependent upon the intracellular pH, with acid pH decreasing, and basic pH increasing, P(o). Internal ATP (2 mM) and Ca(2+) (up to 10(-3) M) had no effect. Channel activity declined irreversibly when the inner side of the patch was exposed to Mg(2+). Kir4.1, Kir4.2 and Kir5.1 mRNAs were all detected in the DCT. The Kir4.1 protein co-localised with the Na(+)-Cl(-) cotransporter, which is specific to the DCT, and was located on basolateral membranes. The DCT K(+) channel differs from other functionally identified renal K(+) channels with regard to its inhibition by spermine and insensitivity to internal ATP and Ca(2+). At the current state of knowledge, the channel is similar to Kir4.1-Kir5.1 and Kir4.2-Kir5.1 heteromeric channels, but not to Kir4.1 or Kir4.2 homomeric channels.

Short term effect of aldosterone on Na,K-ATPase cell surface expression in kidney collecting duct cells

Aldosterone controls extracellular volume and blood pressure by regulating Na+ reabsorption, in particular by epithelia of the distal nephron. A main regulatory site of this transcellular transport is the epithelial sodium channel (ENaC) that mediates luminal Na+ influx. The Na,K-ATPase (Na+ pump) that coordinately extrudes Na+ across the basolateral membrane is known to be regulated by short term aldosterone as well. We now show that in the cortical collecting duct (CCD) from adrenalectomized rats, the increase in Na,K-ATPase activity (approximately 3-fold in 3 h), induced by a single aldosterone injection, can be fully accounted by the increase in Na,K-ATPase cell surface expression (+ 497 +/- 35%). The short term aldosterone action was further investigated in cultured mouse collecting duct principal cells mpkCCD(cl4). Within 2 h, maximal Na,K-ATPase function assessed by Na+ pump current (I(p)) measurements and Na,K-ATPase cell surface expression were increased by 20-50%. Aldosterone did not modify the Na+ dependence of the Na+ pumps and induced transcription- and translation-dependent actions on pump surface expression and current independently of ENaC-mediated Na+ influx. In summary, short term aldosterone directly increases the cell surface expression of pre-existing Na+ pumps in kidney CCD target cells. Thus, aldosterone controls Na+ reabsorption in the short term not only by regulating the apical cell surface expression of ENaC (Loffing, J., Zecevic, M., Feraille, E., Kaissling, B., Asher, C., Rossier, B. C., Firestone, G. L., Pearce, D., and Verrey, F. (2001) Am. J. Physiol. 280, F675-F682) but also by coordinately acting on the basolateral cell surface expression of the Na,K-ATPase.

CFTR disruption impairs cAMP-dependent Cl(-) secretion in primary cultures of mouse cortical collecting ducts

The role of the cystic fibrosis transmembrane conductance regulator (CFTR) in the renal cortical collecting duct (CCD) has not yet been fully elucidated. Here, we investigated the effects of deamino-8-D-arginine vasopressin (dDAVP) and isoproterenol (ISO) on NaCl transport in primary cultured CCDs microdissected from normal [CFTR(+/+)] and CFTR-knockout [CFTR(-/-)] mice. dDAVP stimulated the benzamyl amiloride (BAm)-sensitive transport of Na(+) assessed by the short-circuit current (I(sc)) method in both CFTR(+/+) and CFTR(-/-) CCDs to a very similar degree. Apical addition of 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) or glibenclamide partially inhibited the rise in I(sc) induced by dDAVP and ISO in BAm-treated CFTR(+/+) CCDs, whereas dDAVP, ISO, and NPPB did not alter I(sc) in BAm-treated CFTR(-/-) CCDs. dDAVP stimulated the apical-to-basal flux and, to a lesser extent, the basal-to-apical flux of (36)Cl(-) in CFTR(+/+) CCDs. dDAVP also increased the apical-to-basal (36)Cl(-) flux in CFTR(-/-) CCDs but not the basal-to-apical (36)Cl(-) flux. These results demonstrate that CFTR mediates the cAMP-stimulated component of secreted Cl(-) in mouse CCD.

Cyclosporine reduces basolateral, but not apical, nitric oxide secretion in medullary thick ascending limb cells

Cyclosporine (CsA) reduces nitric oxide (NO) production in medullary thick ascending limb (mTAL) cells. We postulated that CsA affected NO secretion in a vectorial manner in polarized renal epithelial cells. The experiments were performed in a model of mTAL subcultured cells. The expression of iNOS in mTAL cells was confirmed by RT-PCR. The cells were grown on a non-permeable filter. Nitrite was measured by the modified Griess method. Transepithelial resistance was measured to ensure the integrity of the tight junction. CsA (100 ng/ml) reduced NO production by 22% in mTAL cells. The inhibitory effect was limited to the basolateral side (control: 165 +/- 11; plus CsA: 93 +/- 17 nM/10(6) cells, P < 0.001) without affecting apical NO secretion. The transepithelial resistance through the epithelial monolayer remained unchanged in CsA-treated cells. CsA reduced basolateral NO secretion without affecting apical secretion. The results suggest that CsA might affect intrarenal hemodynamics at the peritubular level.

Analysis of the mouse Scnn1a promoter in cortical collecting duct cells and in transgenic mice

We have isolated and characterised the promoter of the mouse Scnn1a (alpha ENaC) gene. Using transient transfections of serial deletion mutants into Scnn1a-expressing cells, we demonstrate that 1.56 kb of 5' upstream sequence is required for cell-specific expression and corticosteroid-mediated regulation. These 5' sequences are not sufficient to drive expression of a lacZ reporter gene or a rat Scnn1a cDNA in transgenic mice, where they failed to rescue Scnn1a deficiency.

Transcriptome of a mouse kidney cortical collecting duct cell line: effects of aldosterone and vasopressin

Aldosterone and vasopressin are responsible for the final adjustment of sodium and water reabsorption in the kidney. In principal cells of the kidney cortical collecting duct (CCD), the integral response to aldosterone and the long-term functional effects of vasopressin depend on transcription. In this study, we analyzed the transcriptome of a highly differentiated mouse clonal CCD principal cell line (mpkCCD(cl4)) and the changes in the transcriptome induced by aldosterone and vasopressin. Serial analysis of gene expression (SAGE) was performed on untreated cells and on cells treated with either aldosterone or vasopressin for 4 h. The transcriptomes in these three experimental conditions were determined by sequencing 169,721 transcript tags from the corresponding SAGE libraries. Limiting the analysis to tags that occurred twice or more in the data set, 14,654 different transcripts were identified, 3,642 of which do not match known mouse sequences. Statistical comparison (at P < 0.05 level) of the three SAGE libraries revealed 34 AITs (aldosterone-induced transcripts), 29 ARTs (aldosterone-repressed transcripts), 48 VITs (vasopressin-induced transcripts) and 11 VRTs (vasopressin-repressed transcripts). A selection of the differentially-expressed, hormone-specific transcripts (5 VITs, 2 AITs and 1 ART) has been validated in the mpkCCD(cl4) cell line either by Northern blot hybridization or reverse transcription-PCR. The hepatocyte nuclear transcription factor HNF-3-alpha (VIT39), the receptor activity modifying protein RAMP3 (VIT48), and the glucocorticoid-induced leucine zipper protein (GILZ) (AIT28) are candidate proteins playing a role in physiological responses of this cell line to vasopressin and aldosterone.

Tissue distribution and subcellular localization of the ClC-5 chloride channel in rat intestinal cells

ClC-5 is the Cl- channel that is mutated in Dent's disease, an X-chromosome-linked disease characterized by low molecular weight proteinuria, hypercalciuria, and kidney stones. It is predominantly expressed in endocytically active renal proximal cells. We investigated whether this Cl- channel could also be expressed in intestinal tissues that have endocytotic machinery. ClC-5 mRNA was detected in the rat duodenum, jejunum, ileum, and colon. Western blot analyses revealed the presence of the 83-kDa ClC-5 protein in these tissues. Indirect immunofluorescence studies showed that ClC-5 was mainly concentrated in the cytoplasm above the nuclei of enterocytes and colon cells. ClC-5 partially colocalized with the transcytosed polymeric immunoglobulin receptor but was not detectable together with the brush-border-anchored sucrase isomaltase. A subfractionation of vesicles obtained by differential centrifugation showed that ClC-5 is associated with the vacuolar 70-kDa H+-ATPase and the small GTPases rab4 and rab5a, two markers of early endosomes. Thus these results indicate that ClC-5 is present in the small intestine and colon of rats and suggest that it plays a role in the endocytotic pathways of intestinal cells.

Vasopressin-stimulated chloride transport in transimmortalized mouse cell lines derived from the distal convoluted tubule and cortical and inner medullary collecting ducts

BACKGROUND

The fine control of NaCl absorption takes place in the distal parts of the renal tubule, but the regulation of Cl(-) transport in this region has not been fully elucidated. We have analysed the effects of dD-arginine vasopressin (dDAVP) on Cl(-) fluxes in cultured mouse distal convoluted tubule (mpkDCT), cortical collecting duct (mpkCCD) and inner medullary collecting duct (mpkIMCD) cell lines.

METHODS

RT-PCR and Western blotting were used to detect the amiloride-sensitive sodium channel (ENaC) and cystic fibrosis transmembrane conductance regulator (CFTR) mRNAs and protein in cultured mpkDCT, mpkCCD and mpkIMCD cells. Cl(-) fluxes were analysed by measuring the short-circuit current (I(sc)) and bidirectional (36)Cl(-) fluxes on confluent cells grown on filters.

RESULTS

All three cell lines expressed ENaC and CFTR and had I(sc) stimulated by dDAVP. The rise in I(sc) caused by dDAVP (10(-8) M) was inhibited by amiloride, and to a lesser extent by 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) in all three cell lines. The dDAVP-dependent I(sc) measured under apical Na(+)-free condition was reduced by Cl(-) channel blockers with a profile (NPPB>glibenclamide>DIDS), similar to that for rat CFTR. dDAVP stimulated the apical-to-basal (36)Cl(-) flux and to a lesser extent the basal-to-apical (36)Cl(-) flux under open-circuit condition in all three cultured cell lines. Adding NPPB to the apical side reduced the basal-to-apical (36)Cl(-) flux but not the opposite (36)Cl(-) flux from dDAVP-treated cells.

CONCLUSION

These results indicate that dDAVP stimulates the bi-directional flux of Cl(-), resulting in net Cl(-)absorption, in these cultured mouse distal and collecting duct cells. I(sc) experiments also suggest the presence of a minor component of electrogenic Cl(-) secretion, possibly mediated by CFTR.

Cyclic AMP increases cell surface expression of functional Na,K-ATPase units in mammalian cortical collecting duct principal cells

Cyclic AMP (cAMP) stimulates the transport of Na(+) and Na,K-ATPase activity in the renal cortical collecting duct (CCD). The aim of this study was to investigate the mechanism whereby cAMP stimulates the Na,K-ATPase activity in microdissected rat CCDs and cultured mouse mpkCCD(c14) collecting duct cells. db-cAMP (10(-3) M) stimulated by 2-fold the activity of Na,K-ATPase from rat CCDs as well as the ouabain-sensitive component of (86)Rb(+) uptake by rat CCDs (1.7-fold) and cultured mouse CCD cells (1.5-fold). Pretreatment of rat CCDs with saponin increased the total Na,K-ATPase activity without further stimulation by db-cAMP. Western blotting performed after a biotinylation procedure revealed that db-cAMP increased the amount of Na,K-ATPase at the cell surface in both intact rat CCDs (1.7-fold) and cultured cells (1.3-fold), and that this increase was not related to changes in Na,K-ATPase internalization. Brefeldin A and low temperature (20 degrees C) prevented both the db-cAMP-dependent increase in cell surface expression and activity of Na,K-ATPase in both intact rat CCDs and cultured cells. Pretreatment with the intracellular Ca(2+) chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid also blunted the increment in cell surface expression and activity of Na,K-ATPase caused by db-cAMP. In conclusion, these results strongly suggest that the cAMP-dependent stimulation of Na,K-ATPase activity in CCD results from the translocation of active pump units from an intracellular compartment to the plasma membrane.

A novel mouse Nedd4 protein suppresses the activity of the epithelial Na+ channel

Liddle's syndrome is a form of inherited hypertension linked to mutations in the genes encoding the epithelial Na+ channel (ENaC). These mutations alter or delete PY motifs involved in protein-protein interactions with a ubiquitin-protein ligase, Nedd4. Here we show that Na+ transporting cells, derived from mouse cortical collecting duct, express two Nedd4 proteins with different structural organization and characteristics of ENaC regulation: 1) the classical Nedd4 (herein referred to as Nedd4-1) containing one amino-terminal C2, three WW, and one HECT-ubiquitin protein ligase domain and 2) a novel Nedd4 protein (Nedd4-2), homologous to Xenopus Nedd4 and comprising four WW, one HECT, yet lacking a C2 domain. Nedd4-2, but not Nedd4-1, inhibits ENaC activity when coexpressed in Xenopus oocytes and this property correlates with the ability to bind to ENaC, as only Nedd4-2 coimmunoprecipitates with ENaC. Furthermore, this interaction depends on the presence of at least one PY motif in the ENaC complex and on WW domains 3 and 4 in Nedd4-2. Thus, these results suggest that the novel suppressor protein Nedd4-2 is the regulator of ENaC and hence a potential susceptibility gene for arterial hypertension.