ZnT1 induces a crosstalk between T-type and L-type calcium channels through interactions with Raf-1 kinase and the calcium channel β2 subunit

ZnT1 is a major zinc transporter that regulates cellular zinc homeostasis. We have previously shown that ZnT1 have additional functions which are independent of its activity as a Zn2+ extruder. These include inhibition of the L type calcium channel (LTCC) through interaction with the auxiliary β-subunit of the LTCC and activation of the Raf-ERK signaling leading to augmented activity of the T-type calcium channel (TTCC). Our findings indicate that ZnT1 increases TTCC activity by enhancing the trafficking of the channel to the plasma membrane. LTCC and TTCC are co-expressed in many tissues and have different functions in a variety of tissues. In the current work, we investigated the effect of the voltage-gated calcium channel (VGCC) β-subunit and ZnT1 on the crosstalk between LTCC and TTCC and their function. Our results indicate that the β-subunit inhibits the ZnT1-induced augmentation of TTCC function. This inhibition correlates with the VGCC β-subunit-dependent reduction in ZnT1-induced activation of Ras-ERK signaling. The effect of ZnT1 is specific as the presence of the β-subunit did not change the effect of Endothelin-1 (ET-1) on TTCC surface expression. These findings document a novel regulatory function of ZnT1 serving as a mediator in the crosstalk between TTCC and LTCC. Overall, we demonstrate that ZnT1 binds and regulates the activity of the β-subunit of VGCC and Raf -1 kinase and modulates surface expression of the LTCC and TTCC catalytic subunits, consequently modulating the activity of these channels.

Zinc transport and the inhibition of the L-type calcium channel are two separable functions of ZnT-1

Traditionally, proteins are considered to perform a single role, be it as an enzyme, a channel, a transporter or as a structural scaffold. However, recent studies have described moonlighting proteins that perform distinct and independent functions; for example, TRPM7 is both an ion channel and a kinase. ZnT-1 is a member of the Carrier Diffusion Facilitator family that is expressed throughout the phylogenetic tree from bacteria to humans. Since its cloning in 1995, ZnT-1 is considered a major extruder of Zn²⁺ based on its capability to protect cells against zinc toxicity. Recently, we reported that ZnT-1 inhibits the L-type calcium channel (LTCC), a major Zn²⁺ and Ca²⁺ entry pathway. Here we show that ZnT-1 is a dual-function protein by demonstrating that its abilities to exchange Zn²⁺/H⁺ and to inhibit the LTCC are independent of each other and are mediated by different parts of the protein. Specifically, mutations in the membrane-spanning helices that render ZnT-1 unable to transport zinc do not prevent it from inhibiting the LTCC. Moreover, a fragment consisting of the intracellular ZnT-1 C-terminal, which lacks all ion-transfer segments, inhibits the LTCC as efficiently as wild-type ZnT-1. Our data therefore indicates that ZnT-1 performs two structurally independent functions related to zinc homeostasis.

APPARENT SELECTIVE NEUTRALITY OF MITOCHONDRIAL DNA SIZE VARIATION: A TEST IN THE DEEP-SEA SCALLOP PLACOPECTEN MAGELLANICUS

Animal mitochondrial DNA (mtDNA) is believed to have evolved under intense selection for economy of the size of the molecule. Among scallop species mtDNA size may vary by a factor of two and among conspecific individuals by as much as 25%. We have examined the possibility that large mtDNA size differences may be associated with fitness in the deep sea scallop Placopecten magellanicus by comparing shell lengths of individuals with different copy numbers of a large mtDNA repeated sequence. Among juvenile cohorts of same age, shell length is known to be a good index of overall fitness in marine bivalves and it is shown here to be affected by differences in nuclear genotype, expressed as the degree of enzyme heterozygosity. We have observed no correlation between shell length and mtDNA length and interpreted this to mean that variation in the size of animal mtDNA is effectively neutral to the forces of natural selection acting on the individual. This type of mtDNA variation must, therefore, be explained in terms of biases in the molecular mechanisms causing expansion or contraction of the molecule, differential replication rates of mtDNA molecules of different size, and the stochastic assortment of mtDNA size classes among individuals.

P2Y6 receptors are involved in mediating the effect of inactivated avian influenza virus H5N1 on IL-6 & CXCL8 mRNA expression in respiratory epithelium

One of the key pathophysiologies of H5N1 infection is excessive proinflammatory cytokine response (cytokine storm) characterized by increases in IFN-β, TNF-α, IL-6, CXCL10, CCL4, CCL2 and CCL5 in the respiratory tract. H5N1-induced cytokine release can occur via an infection-independent mechanism, however, detail of the cellular signaling involved is poorly understood. To elucidate this mechanism, the effect of inactivated (β-propiolactone-treated) H5N1 on the cytokine and chemokine mRNA expression in 16HBE14o- human respiratory epithelial cells was investigated. We found that the inactivated-H5N1 increased mRNA for IL-6 and CXCL8 but not TNF-α, CCL5 or CXCL10. This effect of the inactivated-H5N1 was inhibited by sialic acid receptor inhibitor (α-2,3 sialidase), adenosine diphosphatase (apyrase), P2Y receptor (P2YR) inhibitor (suramin), P2Y6R antagonist (MRS2578), phospholipase C inhibitor (U73122), protein kinase C inhibitors (BIM and Gö6976) and cell-permeant Ca2+ chelator (BAPTA-AM). Inhibitors of MAPK signaling, including of ERK1/2 (PD98059), p38 MAPK (SB203580) and JNK (SP600125) significantly suppressed the inactivated-H5N1-induced mRNA expression of CXCL8. On the other hand, the inactivated-H5N1-induced mRNA expression of IL-6 was inhibited by SB203580, but not PD98059 or SP600125, whereas SN-50, an inhibitor of NF-κB, inhibited the effect of virus on mRNA expression of both of IL-6 and CXCL8. Taken together, our data suggest that, without infection, inactivated-H5N1 induces mRNA expression of IL-6 and CXCL8 by a mechanism, or mechanisms, requiring interaction between viral hemagglutinin and α-2,3 sialic acid receptors at the cell membrane of host cells, and involves activation of P2Y6 purinergic receptors.

Effects of dietary fat profile on gut permeability and microbiota and their relationships with metabolic changes in mice

OBJECTIVE

To distinguish the effects of dietary fat profile on gut parameters and their relationships with metabolic changes and to determine the capacity of n-3 fatty acids to modify gut variables in the context of diet-induced metabolic dysfunctions.

METHODS

Mice received control or high-fat diets emphasizing saturated (HFD-sat), n-6 (HFD-n6), or n-3 (HFD-n3) fatty acids for 8 weeks. In another cohort, mice that were maintained on HFD-sat received n-3-rich fish oil or resolvin D1 supplementation.

RESULTS

HFD-sat and HFD-n6 induced similar weight gain, but only HFD-sat increased index of insulin resistance (HOMA-IR), colonic permeability, and mesenteric fat inflammation. Hydrogen sulfide-producing bacteria were one of the major groups driving the diet-specific changes in gut microbiome, with the overall microbial profile being associated with changes in body weight, HOMA-IR, and gut permeability. In mice maintained on HFD-sat, fish oil and resolvin D1 restored barrier function and reduced inflammation in the colon but were unable to normalize HOMA-IR.

CONCLUSIONS

Different dietary fat profiles led to distinct intestinal and metabolic outcomes that are independent of obesity. Interventions targeting inflammation successfully restored gut health but did not reverse systemic aspects of diet-induced metabolic dysfunction, implicating separation between gut dysfunctions and disease-initiating and/or -maintaining processes.

ZnT-1 extrudes zinc from mammalian cells functioning as a Zn(2+)/H(+) exchanger

ZnT-1 is a Cation Diffusion Facilitator (CDF) family protein, and is present throughout the phylogenetic tree from bacteria to humans. Since its original cloning in 1995, ZnT-1 has been considered to be the major Zn(2+) extruding transporter, based on its ability to protect cells against zinc toxicity. However, experimental evidence for ZnT-1 induced Zn(2+) extrusion was not convincing. In the present study, based on the 3D crystal structure of the ZnT-1 homologue, YiiP, that predicts a homodimer that utilizes the H(+) electrochemical gradient to facilitate Zn(2+) efflux, we demonstrate ZnT-1 dependent Zn(2+) efflux from HEK 293T cells using FluoZin-3 and Fura 2 by single cell microscope based fluorescent imaging. ZnT-1 facilitates zinc efflux in a sodium-independent, pH-driven and calcium-sensitive manner. Moreover, substitution of two amino acids in the putative zinc binding domain of ZnT-1 led to nullification of Zn(2+) efflux and rendered the mutated protein incapable of protecting cells against Zn(2+) toxicity. Our results demonstrate that ZnT-1 extrudes zinc from mammalian cells by functioning as a Zn(2+)/H(+) exchanger.

Changing expression of chloride channels during preimplantation mouse development

Plasma membrane chloride channels (ClCs) play important roles in a broad range of cellular processes including cell volume regulation, proliferation, and transepithelial transport, all of which are critical during preimplantation embryonic development. In this study, the molecular and functional expression of voltage-gated ClCs was analyzed throughout preimplantation development of the mouse conceptus. mRNA transcripts for allClcngenes were detected. OnlyClcn1mRNA showed differential expression in the blastocyst, being detected in the trophectoderm but not in the inner cell mass. CLCN3 protein was detected at low levels in the cytoplasm and plasma membrane in 4-cell embryos and was localized to the apical plasma membrane of the trophoblasts in the blastocyst. Whole-cell patch-clamp recordings demonstrated the presence of a DIDS-sensitive, outwardly rectifying Cl−current throughout development, with this conductance being large at the 1-cell, morula and blastocyst stages. A second DIDS-insensitive Cl−current, which was inactivated by membrane depolarization, was present in cells differentiating into the trophoblast lineage and during blastocyst expansion. Inhibition of the DIDS-sensitive current and the DIDS-insensitive current, with 9-AC, prevented blastocyst expansion.

Increased gut permeability and microbiota change associate with mesenteric fat inflammation and metabolic dysfunction in diet-induced obese mice

We investigated the relationship between gut health, visceral fat dysfunction and metabolic disorders in diet-induced obesity. C57BL/6J mice were fed control or high saturated fat diet (HFD). Circulating glucose, insulin and inflammatory markers were measured. Proximal colon barrier function was assessed by measuring transepithelial resistance and mRNA expression of tight-junction proteins. Gut microbiota profile was determined by 16S rDNA pyrosequencing. Tumor necrosis factor (TNF)-α and interleukin (IL)-6 mRNA levels were measured in proximal colon, adipose tissue and liver using RT-qPCR. Adipose macrophage infiltration (F4/80⁺) was assessed using immunohistochemical staining. HFD mice had a higher insulin/glucose ratio (P = 0.020) and serum levels of serum amyloid A3 (131%; P = 0.008) but reduced circulating adiponectin (64%; P = 0.011). In proximal colon of HFD mice compared to mice fed the control diet, transepithelial resistance and mRNA expression of zona occludens 1 were reduced by 38% (P<0.001) and 40% (P = 0.025) respectively and TNF-α mRNA level was 6.6-fold higher (P = 0.037). HFD reduced Lactobacillus (75%; P<0.001) but increased Oscillibacter (279%; P = 0.004) in fecal microbiota. Correlations were found between abundances of Lactobacillus (r = 0.52; P = 0.013) and Oscillibacter (r = −0.55; P = 0.007) with transepithelial resistance of the proximal colon. HFD increased macrophage infiltration (58%; P = 0.020), TNF-α (2.5-fold, P<0.001) and IL-6 mRNA levels (2.5-fold; P = 0.008) in mesenteric fat. Increased macrophage infiltration in epididymal fat was also observed with HFD feeding (71%; P = 0.006) but neither TNF-α nor IL-6 was altered. Perirenal and subcutaneous adipose tissue showed no signs of inflammation in HFD mice. The current results implicate gut dysfunction, and attendant inflammation of contiguous adipose, as salient features of the metabolic dysregulation of diet-induced obesity.

Frog skin epithelium: electrolyte transport and chytridiomycosis

One unique physiological characteristic of frogs is that their main route for intake of water is across the skin. In these animals, the skin acts in concert with the kidney and urinary bladder to maintain electrolyte homeostasis. Water absorption across the skin is driven by the osmotic gradient that develops as a consequence of solute transport. Our recent study demonstrated that chytridiomycosis, an infection of amphibian skin by the fungal pathogen, Batrachochytrium dendrobatidis, inhibits epithelial Na(+) channels, attenuating Na(+) absorption through the skin. In frogs that become severely affected by this fungus, systemic depletion of Na(+), K(+) and Cl(-) is thought to cause deterioration of cardiac electrical function, leading to cardiac arrest. Here we review the ion transport mechanisms of frog skin, and discuss the effect of chytridiomycosis on these mechanisms.

Regulation of the epithelial Na+ channel by the RH domain of G protein-coupled receptor kinase, GRK2, and Galphaq/11

The G protein-coupled receptor kinase (GRK2) belongs to a family of protein kinases that phosphorylates agonist-activated G protein-coupled receptors, leading to G protein-receptor uncoupling and termination of G protein signaling. GRK2 also contains a regulator of G protein signaling homology (RH) domain, which selectively interacts with α-subunits of the Gq/11 family that are released during G protein-coupled receptor activation. We have previously reported that kinase activity of GRK2 up-regulates activity of the epithelial sodium channel (ENaC) in a Na(+) absorptive epithelium by blocking Nedd4-2-dependent inhibition of ENaC. In the present study, we report that GRK2 also regulates ENaC by a mechanism that does not depend on its kinase activity. We show that a wild-type GRK2 (wtGRK2) and a kinase-dead GRK2 mutant ((K220R)GRK2), but not a GRK2 mutant that lacks the C-terminal RH domain (ΔRH-GRK2) or a GRK2 mutant that cannot interact with Gαq/11/14 ((D110A)GRK2), increase activity of ENaC. GRK2 up-regulates the basal activity of the channel as a consequence of its RH domain binding the α-subunits of Gq/11. We further found that expression of constitutively active Gαq/11 mutants significantly inhibits activity of ENaC. Conversely, co-expression of siRNA against Gαq/11 increases ENaC activity. The effect of Gαq on ENaC activity is not due to change in ENaC membrane expression and is independent of Nedd4-2. These findings reveal a novel mechanism by which GRK2 and Gq/11 α-subunits regulate the activity ENaC.

Rotavirus toxin NSP4 induces diarrhea by activation of TMEM16A and inhibition of Na+ absorption

Rotavirus infection is the most frequent cause for severe diarrhea in infants, killing more than 600,000 every year. The nonstructural protein NSP4 acts as a rotavirus enterotoxin, inducing secretory diarrhea without any structural organ damage. Electrolyte transport was assessed in the colonic epithelium from pups and adult mice using Ussing chamber recordings. Western blots and immunocytochemistry was performed in intestinal tissues from wild-type and TMEM16A knockout mice. Ion channel currents were recorded using patch clamp techniques. We show that the synthetic NSP4(114-135) peptide uses multiple pro-secretory pathways to induce diarrhea, by activating the recently identified Ca2+ -activated Cl- channel TMEM16A, and by inhibiting Na+ absorption by the epithelial Na+ channel ENaC and the Na+ /glucose cotransporter SGLT1. Activation of secretion and inhibition of Na+ absorption by NSP4(114-135), respectively, could be potently suppressed by wheat germ agglutinin which probably competes with NSP4(114-135) for binding to an unknown glycolipid receptor. The present paper gives a clue as to mechanisms of rotavirus-induced diarrhea and suggests wheat germ agglutinin as a simple and effective therapy.

Reduced activity of the epithelial sodium channel in malaria-induced pulmonary oedema in mice

Lung complications during malaria infection can range from coughs and impairments in gas transfer to the development of acute respiratory distress syndrome (ARDS). Infecting C57BL/6 mice with Plasmodium berghei K173 strain (PbK) resulted in pulmonary oedema, capillaries congested with leukocytes and infected red blood cells (iRBCs), and leukocyte infiltration into the lungs. This new model of malaria-associated lung pathology, without any accompanying cerebral complications, allows the investigation of mechanisms leading to the lung disease. The activity of the amiloride-sensitive epithelial sodium channel (ENaC) in alveolar epithelial cells is decreased by several respiratory tract pathogens and this is suggested to contribute to pulmonary oedema. We show that PbK, a pathogen that remains in the circulation, also decreased the activity and expression of ENaC, suggesting that infectious agents can have indirect effects on ENaC activity in lung epithelial cells. The reduced ENaC activity may contribute to the pulmonary oedema induced by PbK malaria.

Negative regulation of Ca(2+) influx during P2Y(2) purinergic receptor activation is mediated by Gbetagamma-subunits

We have previously reported that P2Y(2) purinoceptors and muscarinic M(3) receptors trigger Ca(2+) responses in HT-29 cells that differ in their timecourse, the Ca(2+) response to P2Y(2) receptor activation being marked by a more rapid decline of intracellular Ca(2+) concentration ([Ca(2+)](i)) after the peak response and that this rapid decline of [Ca(2+)](i) was slowed in cells expressing heterologous beta-adrenergic receptor kinase (betaARK). In the present study, we demonstrate that, during P2Y(2) receptor activation, betaARK expression increases the rate of Gd(3+)-sensitive Mn(2+) influx, a measure of the rate of store-operated Ca(2+) entry from the extracellular space, during P2Y(2) activation and that this effect of betaARK is mimicked by exogenous alpha-subunits of G(q), G(11) and G(i2). The effect of betaARK on the rate of Mn(2+) influx is thus attributable to its ability to scavenge G protein betagamma-subunits released during activation of P2Y(2) receptor. We further find that the effect of betaARK on the rate of Mn(2+) influx during P2Y(2) receptor activation can be overcome by arachidonic acid. In addition, the UTP-induced Mn(2+) influx rate was significantly increased by inhibitors of phospholipase A(2) (PLA(2)) and an siRNA directed against PLA(2)beta, but not by an siRNA directed against PLA(2)alpha or by inhibitors of arachidonic acid metabolism. These findings provide evidence for the existence of a P2Y(2) receptor-activated signalling system that acts in parallel with depletion of intracellular Ca(2+) stores to inhibit Ca(2+) influx across the cell membrane. This signalling process is mediated via Gbetagamma and involves PLA(2)beta and arachidonic acid.

Purinergic regulation of the epithelial Na+ channel

1. The epithelial Na(+) channel (ENaC) is a major conductive pathway that transports Na(+) across the apical membrane of the distal nephron, the respiratory tract, the distal colon and the ducts of exocrine glands. The ENaC is regulated by hormonal and humoral factors, including extracellular nucleotides that are available from the epithelial cells themselves. 2. Extracellular nucleotides, via the P2Y2 receptors (P2Y2Rs) at the basolateral and apical membrane of the epithelia, trigger signalling systems that inhibit the activity of the ENaC and activate Ca(2+) -dependent Cl(-) secretion. 3. Recent data from our laboratory suggest that stimulation of the P2Y2Rs at the basolateral membrane inhibits ENaC activity by a signalling mechanism that involves G beta gamma subunits freed from a pertussis toxin (PTX)-sensitive G-protein and phospholipase C (PLC) beta 4. A similar signalling mechanism is also partially responsible for inhibition of the ENaC during activation of apical P2Y2Rs. 4. Stimulation of apical P2Y2Rs also activates an additional signalling mechanism that inhibits the ENaC and involves the activated Galpha subunit of a PTX-insensitive G-protein and activation of an unidentified PLC. The effect of this PTX-insensitive system requires the activity of the basolateral Na(+)/K(+)/2Cl(-) cotransporter.

The activity of the epithelial sodium channels is regulated by caveolin-1 via a Nedd4-2-dependent mechanism

It has recently been shown that the epithelial Na(+) channel (ENaC) is compartmentalized in caveolin-rich lipid rafts and that pharmacological depletion of membrane cholesterol, which disrupts lipid raft formation, decreases the activity of ENaC. Here we show, for the first time, that a signature protein of caveolae, caveolin-1 (Cav-1), down-regulates the activity and membrane surface expression of ENaC. Physical interaction between ENaC and Cav-1 was also confirmed in a coimmunoprecipitation assay. We found that the effect of Cav-1 on ENaC requires the activity of Nedd4-2, a ubiquitin protein ligase of the Nedd4 family, which is known to induce ubiquitination and internalization of ENaC. The effect of Cav-1 on ENaC requires the proline-rich motifs at the C termini of the beta- and gamma-subunits of ENaC, the binding motifs that mediate interaction with Nedd4-2. Taken together, our data suggest that Cav-1 inhibits the activity of ENaC by decreasing expression of ENaC at the cell membrane via a mechanism that involves the promotion of Nedd4-2-dependent internalization of the channel.

Regulation of epithelial Na+ channels by aldosterone: role of Sgk1

1. The epithelial sodium channel (ENaC) is tightly regulated by hormonal and humoral factors, including cytosolic ion concentration and glucocorticoid and mineralocorticoid hormones. Many of these regulators of ENaC control its activity by regulating its surface expression via neural precursor cell-expressed developmentally downregulated (gene 4) protein (Nedd4-2). 2. During the early phase of aldosterone action, Nedd4-2-dependent downregulation of ENaC is inhibited by the serum- and glucocorticoid-induced kinase 1 (Sgk1). 3. Sgk1 phosphorylates Nedd4-2. Subsequently, phosphorylated Nedd4-2 binds to the 14-3-3 protein and, hence, reduces binding of Nedd4-2 to ENaC. 4. Nedd4-2 is also phosphorylated by protein kinase B (Akt1). Both Sgk1 and Akt1 are part of the insulin signalling pathway that increases transepithelial Na(+) absorption by inhibiting Nedd4-2 and activating ENaC.

(NDRG2) Stimulates Amiloride-sensitive Na+ Currents in Xenopus laevis Oocytes and Fisher Rat Thyroid Cells

Regulation of the epithelial sodium channel (ENaC) is highly complex and may involve several aldosterone-induced regulatory proteins. The N-Myc downstream-regulated gene 2 (NDRG2) has been identified as an early aldosterone-induced gene. Therefore, we hypothesized that NDRG2 may affect ENaC function. To test this hypothesis we measured the amiloride-sensitive (2 microm) whole cell current (DeltaI(ami)) in Xenopus laevis oocytes expressing ENaC alone or co-expressing ENaC and NDRG2. Co-expression of NDRG2 significantly increased DeltaI(ami) in some, but not, all batches of oocytes tested. An inhibitory effect of NDRG2 was never observed. Using a chemiluminescence assay we demonstrated that the NDRG2-induced increase in ENaC currents was accompanied by a similar increase in channel surface expression. The stimulatory effect of NDRG2 was preserved in oocytes maintained in a low sodium bath solution to prevent sodium feedback inhibition. These findings suggest that the stimulatory effect of NDRG2 is independent of sodium feedback regulation. Furthermore, the stimulatory effect of NDRG2 on ENaC was at least in part additive to that of Sgk1. A short isoform of NDRG2 also stimulated DeltaI(ami). Overexpression of NDRG2 and ENaC in Fisher rat thyroid cells confirmed the stimulatory effect of NDRG2 on ENaC-mediated short-circuit current (I(SC-ami)). In addition, small interference RNA against NDRG2 largely reduced I(SC-ami) in Fisher rat thyroid cells. Our results indicate that NDRG2 is a likely candidate to contribute to aldosterone-mediated ENaC regulation.

Akt mediates the effect of insulin on epithelial sodium channels by inhibiting Nedd4-2

The epithelial sodium channel (ENaC) plays an important role in transepithelial Na(+) absorption; hence its function is essential for maintaining Na(+) and fluid homeostasis and regulating blood pressure. Insulin is one of the hormones that regulates activity of ENaC. In this study, we investigated the contribution of two related protein kinases, Akt (also known as protein kinase B) and the serum- and glucocorticoid-dependent kinase (Sgk), on insulin-induced ENaC activity in Fisher rat thyroid cells expressing ENaC. Overexpression of Akt1 or Sgk1 significantly increased ENaC activity, whereas expression of a dominant-negative construct of Akt1, Akt1(K179M), decreased basal activity of ENaC. Inhibition of the endogenous expression of Akt1 and Sgk1 by short interfering RNA not only inhibited ENaC but also disrupted the stimulatory effect on ENaC of insulin and of the downstream effectors of insulin, phosphatidylinositol 3-kinase and PDK1. Conversely, overexpression of Akt1 or Sgk1 increased expression of ENaC at the cell membrane and overcame the inhibitory effect of Nedd4-2 on ENaC. Furthermore, mutation of consensus phosphorylation sites on Nedd4-2 for Akt1 and Sgk1, Ser(342) and Ser(428), completely abolished the inhibitory effect of Sgk1 and Akt1 on Nedd4-2 action. Together these data suggest that both Akt and Sgk are components of an insulin signaling pathway that increases Na(+) absorption by up-regulating membrane expression of ENaC via a regulatory system that involves inhibition of Nedd4-2.

GRK2 interacts with and phosphorylates Nedd4 and Nedd4-2

Epithelial Na(+) channels (ENaC) mediate the transport of sodium (Na) across epithelia in the kidney, gut, and lungs and are required for blood pressure regulation. They are inhibited by ubiquitin protein ligases, such as Nedd4 and Nedd4-2, which bind to proline-rich motifs (PY motifs) present in the C-termini of ENaC subunits. Loss of inhibition leads to hypertension. ENaC channels are maintained in the active state by G-protein-coupled receptor kinase 2 (GRK2), an enzyme implicated in the development of essential hypertension. Here, we report that GRK2 interacts not only with ENaC, but also with both Nedd4 and Nedd4-2. Additionally, GRK2 is capable of phosphorylating both Nedd4 and Nedd4-2 at multiple sites. Of possible significance is the phosphorylation of the threonine at position 466 in Nedd4, which is located in the area of the ww3 domain that binds ENaC. These results support and extend the role of GRK2 in sodium transport regulation.

Inhibition of airway Na+ transport by respiratory syncytial virus

In previous studies, we have shown that two major respiratory pathogens, influenza virus and parainfluenza virus, produce acute alterations in ion transport upon contacting the apical membrane of the respiratory epithelium. In the present study, we examine the effects on ion transport by the mouse tracheal epithelium of a third major respiratory pathogen, respiratory syncytial virus (RSV). RSV infections are associated with fluid accumulation in the respiratory tract and cause illnesses that range in severity from rhinitis, sinusitis, otitis media, and bronchitis to bronchiolitis and pneumonia. We find that within minutes of RSV contacting the apical membrane; it inhibits amiloride-sensitive Na+ transport by the epithelium. This effect is mediated by protein kinase C and is reproduced by recombinant viral F (fusion) protein. Since this inhibition is not accompanied by any alteration in the epithelial responses to carbachol or to forskolin plus 3-isobutyl-1-methylxanthine (IBMX), it is not due to a nonspecific toxic action of the virus. The inhibition also appears to require Toll-like receptor 4 and the presence of asialogangliosides in the apical membrane. Since the concentration range over which this inhibition is observed (10(2) to 10(5) PFU/ml) is comparable to the viral concentrations observed in clinical and experimental RSV infections, it seems likely that direct inhibition by the virus of epithelial Na+ transport may contribute to the fluid accumulation that is observed in RSV infections.

Scoring of predicted GRK2 phosphorylation sites in Nedd4-2

MOTIVATION

Epithelial Na(+) channels (ENaC) mediate the transport of sodium (Na) across epithelia in the kidney, gut and lungs and are required for blood pressure regulation. They are inhibited by ubiquitin protein ligases, such as Nedd4-2. These ligases bind to proline-rich motifs (PY motifs) present in the C-termini of ENaC subunits. Loss of this inhibition leads to hypertension. We have previously reported that ENaC channels are maintained in the active state by the G protein coupled receptor kinase, GRK2. The enzyme has been implicated in the development of essential hypertension [R. D. Feldman (2002) Mol. Pharmacol., 61, 707-709]. Additional findings in our lab pointed towards a possible role for GRK2 in the phosphorylation and inactivation of Nedd4-2.

RESULTS

We have predicted GRK2 phosphorylation sites on Nedd4-2 by combining sequence analysis, homology modeling and surface accessibility calculations. A total of 24 potential phosphorylation sites were predicted by sequence analysis. Of these, 16 could be modeled using homology modeling and 6 of these were found to have sufficient surface exposure to be accessible to the GRK2 enzyme responsible for the phosphorylation of Nedd4-2. The method provides an ordered list of the most probable GRK2 phosphorylation sites on Nedd4-2 providing invaluable guidance to future experimental studies aimed at mutating certain Nedd4-2 residues in order to prevent phosphorylation by GRK2. The method developed could be applied in a wide variety of biological applications involving the binding of one molecule to a protein. The relative effectiveness of the technique is determined mainly by the quality of the homology model built for the protein of interest.

CONTACT

jarthur@med.usyd.edu.au

Stimulation of the epithelial sodium channel (ENaC) by the serum- and glucocorticoid-inducible kinase (Sgk) involves the PY motifs of the channel but is independent of sodium feedback inhibition

The epithelial sodium channel (ENaC) is the major mediator of sodium transport across the apical membranes of the distal nephron, the distal colon, the respiratory tract and the ducts of exocrine glands. It is subject to feedback inhibition by increased intracellular Na+, a regulatory system wherein the ubiquitin protein ligases, Nedd4 and Nedd4-2, bind to conserved PY motifs in the C-termini of ENaC and inactivate the channel. It has been proposed recently that the kinase Sgk activates the channel as a consequence of phosphorylating Nedd4-2, thus preventing it from inhibiting the channels. This proposal predicts that Sgk should interfere with Na+ feedback regulation of ENaC. We have tested this prediction in Xenopus laevis oocytes and in mouse salivary duct cells and found that in neither system did increased activity of Sgk interrupt Na+ feedback inhibition of ENaC. We found, however, that Sgk stimulation was largely abolished in oocytes expressing ENaC channels with C-terminal truncations or mutated PY motifs. We were also unable to confirm that Sgk directly interacts with Nedd4-2 in vitro. We conclude that the stimulatory effect of Sgk on ENaC requires the presence of the channel's PY motifs, but it is not due to the interruption of Na+ feedback regulation.

The ubiquitin-protein ligases Nedd4 and Nedd4-2 show similar ubiquitin-conjugating enzyme specificities

Nedd4 and Nedd4-2 are closely related HECT-type ubiquitin-protein ligases (E3) implicated in the regulation of a number of proteins and pathways. Given the close homology between these E3 enzymes it would be predicted that a conserved ubiquitin-conjugating enzyme (E2) specificity exists between the two proteins. However, E2 specificities for Nedd4 and Nedd4-2 are not well established. In the present studies we aimed at clarifying the E2-specificities of Nedd4 and Nedd4-2 using in vitro ubiquitination assays. We demonstrate strong substrate ubiquitination in the presence of UbcH5b by both Nedd4 and Nedd4-2. We also found that Ube2e3, an E2 previously shown to be used by Nedd4-2, is used less efficiently than UbcH5b. Our results suggest that for optimal ubiquitination Nedd4 and Nedd4-2 require the same E2 enzymes.

Nedd4-2 Functionally Interacts with ClC-5

Constitutive albumin uptake by the proximal tubule is achieved by a receptor-mediated process in which the Cl(-) channel, ClC-5, plays an obligate role. Here we investigated the functional interaction between ClC-5 and ubiquitin ligases Nedd4 and Nedd4-2 and their role in albumin uptake in opossum kidney proximal tubule (OK) cells. In vivo immunoprecipitation using an anti-HECT antibody demonstrated that ClC-5 bound to ubiquitin ligases, whereas glutathione S-transferase pull-downs confirmed that the C terminus of ClC-5 bound both Nedd4 and Nedd4-2. Nedd4-2 alone was able to alter ClC-5 currents in Xenopus oocytes by decreasing cell surface expression of ClC-5. In OK cells, a physiological concentration of albumin (10 mug/ml) rapidly increased cell surface expression of ClC-5, which was also accompanied by the ubiquitination of ClC-5. Albumin uptake was reduced by inhibiting either the lysosome or proteasome. Total levels of Nedd4-2 and proteasome activity also increased rapidly in response to albumin. Overexpression of ligase defective Nedd4-2 or knockdown of endogenous Nedd4-2 with small interfering RNA resulted in significant decreases in albumin uptake. In contrast, pathophysiological concentrations of albumin (100 and 1000 mug/ml) reduced the levels of ClC-5 and Nedd4-2 and the activity of the proteasome to the levels seen in the absence of albumin. These data demonstrate that normal constitutive uptake of albumin by the proximal tubule requires Nedd4-2, which may act via ubiquitination to shunt ClC-5 into the endocytic pathway.

Acute effects of parainfluenza virus on epithelial electrolyte transport

Parainfluenza viruses are important causes of respiratory disease in both children and adults. In particular, they are the major cause of the serious childhood illness croup (laryngotracheobronchitis). The infections produced by parainfluenza viruses are associated with the accumulation of ions and fluid in the respiratory tract. It is not known, however, whether this accumulation is because of a direct effect of the viruses on ion and fluid transport by the respiratory epithelium. Here we show that a model parainfluenza virus (the Sendai virus), in concentrations observed during respiratory infections, activates Cl- secretion and inhibits Na+ absorption across the tracheal epithelium. It does so by binding to a neuraminidase-insensitive glycolipid, possibly asialo-GM1, triggering the release of ATP, which then acts in an autocrine fashion on apical P2Y receptors to produce the observed changes in ion transport. These findings indicate that fluid accumulation in the respiratory tract associated with parainfluenza virus infection is attributable, at least in part, to direct effects of the virus on ion transport by the respiratory epithelium.

The kinase Grk2 regulates Nedd4/Nedd4-2-dependent control of epithelial Na+ channels

Epithelial Na(+) channels mediate the transport of Na across epithelia in the kidney, gut, and lungs and are required for blood pressure regulation. They are inhibited by ubiquitin protein ligases, such as Nedd4 and Nedd4-2, with loss of this inhibition leading to hypertension. Here, we report that these channels are maintained in the active state by the G protein-coupled receptor kinase, Grk2, which has been previously implicated in the development of essential hypertension. We also show that Grk2 phosphorylates the C terminus of the channel beta subunit and renders the channels insensitive to inhibition by Nedd4-2. This mechanism has not been previously reported to regulate epithelial Na(+) channels and provides a potential explanation for the observed association of Grk2 overactivity with hypertension. Here, we report a G protein-coupled receptor kinase regulating a membrane protein other than a receptor and provide a paradigm for understanding how the interaction between membrane proteins and ubiquitin protein ligases is controlled.

Regulation of Neuronal Voltage-gated Sodium Channels by the Ubiquitin-Protein Ligases Nedd4 and Nedd4-2

Nedd4 and Nedd4-2 are ubiquitin-protein ligases known to regulate a number of membrane proteins including receptors and ion transporters. Regulation of the epithelial Na(+) channel by Nedd4 and Nedd4-2 is mediated via interactions between the PY motifs of the epithelial sodium channel subunits and the Nedd4/Nedd4-2 WW domains. This example serves as a model for the regulation of other PY motif-containing ion channels by Nedd4 and Nedd4-2. We found that the carboxyl termini of the six voltage-gated Na(+) (Na(v)) channels contain typical PY motifs (PPXY), and a further Na(v) contains a PY motif variant (LPXY). Not only did we demonstrate by Far-Western analysis that Nedd4 and Nedd4-2 interact with the PY motif-containing Na(v) channels, but we also showed that these channels have conserved WW domain binding specificity. We further showed that the carboxyl termini fusion proteins of one central nervous system and one peripheral nervous system-derived Na(+) channel (Na(v)1.2 and Na(v)1.7, respectively) are readily ubiquitinated by Nedd4-2. In Xenopus oocytes, Nedd4-2 strongly inhibited the activities of all three Na(v)s (Na(v)1.2, Na(v)1.7, and Na(v)1.8) tested. Interestingly, Nedd4 suppressed the activity of Na(v)1.2 and Na(v)1.7 but was a poor inhibitor of Na(v)1.8. Our results provide evidence that Nedd4 and Nedd4-2 are likely to be key regulators of specific neuronal Na(v) channels in vivo.

Expression and role of the ether-à-go-go-related (MERG1A) potassium-channel protein during preimplantation mouse development

Potassium channels play important roles in many cellular processes, including cell-cycle progression and cell differentiation. In the present study, we investigated the pattern of expression of the mouse ether-à-go-go-related (KCNH2; MERG1A) potassium channel during mouse embryogenic development. Analysis by reverse transcription-polymerase chain reaction revealed maternal MERG1A transcripts until the late 2-cell stage of development, after which MERG1A expression from the zygotic genome was low until the 8-cell stage, then rose in the morula, but was low in trophoblast compared to inner cell mass cells. A trophoblast stem cell line also was shown to express MERG1A mRNA. Immunoblotting of oocytes, blastocysts, and the trophoblast stem cell line revealed different posttranslationally processed forms of MERG1A. Immunofluorescence analysis showed that the subcellular localization of MERG1A varied at different stages of the embryogenic cell cycle. In addition, MERG1A protein levels increased following compaction at the 8-cell stage, and its distribution became polarized. This relocalization of MERG1A was affected by treatment with specific inhibitors of ether-à-go-go-related gene (ERG)-channel function and of actin polymerization. Puromycin treatment of morulae indicated that membrane-associated MERG1A had a half-life of greater than 24 h. The ERG-specific inhibitor E-4031 reduced the incidence of blastocyst formation and the number of cells per blastocyst. These results show that MERG1A is developmentally regulated and suggest that it might play a role in early mouse embryogenic development.

Insulin secretagogues, but not glucose, stimulate an increase in [Ca2+]i in the fetal human and porcine beta-cell

Fetal pancreatic beta-cells release insulin poorly in response to glucose; however, the cellular mechanism for this is unknown. By using fura-2 to measure changes in the cytoplasmic free Ca(2+) concentration in beta-cells, we examined human/porcine fetal islet-like cell clusters (ICCs) and human adult islets for the presence of functional K(+)(ATP) and voltage-activated Ca(2+) ion channels. The effects of glucose, glyceraldehyde, leucine, KCl, and the channel effectors glipizide and BAY K8644 were studied. In fetal human/porcine ICCs and adult islets, KCl, glipizide, and BAY K8644 increased [Ca(2+)](i). Both glucose and glyceraldehyde increased [Ca(2+)](i) in islets but had no effect on ICCs. Leucine increased [Ca(2+)](i) in islets and porcine but not human ICCs. We hypothesize that the beneficial effect of leucine in fetal porcine, but not human ICCs, is attributable to time-dependent maturation of the beta-cells, because porcine ICCs examined were at 87% of the gestational period, and human ICCs were at 42%. Our data demonstrate that both K(+)(ATP) and voltage-activated Ca(2+) channels, required for glucose-stimulated increase in [Ca(2+)](i), are functional early in gestation. This suggests that the cause of the immaturity of fetal human/porcine beta-cells is at a more proximal step of glucose-induced metabolism than the channels on the cell surface.

Use of adenoviruses to study isoform specificity of G-protein-receptor-coupled Ca2+ signaling in intact epithelial cells

It is firmly established that the activation of many heptahelical receptors by extracellular agonists leads to the activation of effectors such as phospholipase Cbeta (PLCbeta), the subsequent production of inositol-1,4,5-trisphosphate (IP3), and a resultant increase in intracellular free Ca2+. Heterotrimeric G-proteins have a critical role in transducing the signal from the heptahelical receptor to PLCbeta and in determining the specificity and duration of the cellular responses. There remain, however, a number of areas of uncertainty regarding the exact mechanisms involved in regulating G-protein-mediated receptor-effector coupling in different cell types. For example, the molecular identity of the G-protein involved and the degree of isoform specificity among G-proteins of the same family and their receptors remains unclear. It is also not known in many cell types whether it is the alpha- or the betagamma-subunits of these G-proteins that activate PLCbeta. In order to address these issues, we have used replication-deficient adenoviruses as a tool to deliver, into intact epithelial cells, transgenes coding for proteins involved in G-protein-coupled receptor signaling pathways.

Patch-clamp studies on epithelial sodium channels in salivary duct cells

Mouse mandibular salivary duct cells contain an amiloride-sensitive Na+ current and express all three subunits of the epithelial Na+ channel, ENaC. This amiloride-sensitive Na+ current is subject to feedback regulation by intracellular Na+ and we have previously demonstrated that this regulation is mediated by an ubiquitin-protein ligase, which we identified as Nedd4. The evidence supporting this identification is as follows: (1) antibodies raised against murine Nedd4 block Na+ feedback inhibition; (2) a mutant of murine Nedd4 containing the WW domains but no HECT domain (ubiquitin-protein ligase) blocks Na+ feedback inhibition; and (3) Nedd4 is expressed in mouse mandibular salivary duct cells. In the present studies, we have used whole-cell patch-clamp methods to further investigate the mechanisms by which ubiquitin-protein ligases regulate the amiloride-sensitive Na+ conductance in mouse salivary duct cells. In particular, we have examined the possibility that the ubiquitin-protein ligase, KIAA0439, which is closely related to Nedd4, may mediate Na+ feedback control of amiloride-sensitive Na+ channels. Furthermore, we have attempted to define the mechanism by which ubiquitin-protein ligases inhibit Na+ channels. We have found that KIAA0439 is expressed in mouse mandibular ducts and interacts with the PY motifs of the alpha-, beta-, and gamma-subunits of ENaC in vitro. Furthermore, in whole-cell patch-clamp studies, a glutathione-S-transferase (GST)-fusion protein containing the WW motifs of human KIAA0439 was able to inhibit feedback regulation of the amiloride-sensitive Na+ current by intracellular Na+. We also examined whether GST-fusion proteins containing the C-termini of the alpha-, beta-, and gamma-subunits of ENaC are able to interrupt Na+ feedback regulation of the amiloride-sensitive Na+ current. We found that the C-termini of the beta- and gamma-subunits were able to do so, whereas the C-terminus of the alpha-subunit was not. We conclude that KIAA0439 is, together with Nedd4, a potential mediator of the control of epithelial Na+ channels in salivary duct cells by intracellular Na+. We further conclude that ubiquitin-protein ligases interact with the Na+ channels through the C-termini of the beta- and gamma-subunits of the Na+ channels.

The role of individual Nedd4-2 (KIAA0439) WW domains in binding and regulating epithelial sodium channels

The amiloride-sensitive epithelial sodium channel (ENaC) is essential for fluid and electrolyte homeostasis. ENaC consists of alpha, beta, and gamma subunits, each of which contains a PPxY motif that interacts with the WW domains of the ubiquitin-protein ligases Nedd4 and Nedd4-2. Disruption of this interaction, as in Liddle's syndrome in which mutations delete or alter the PPxY motif of either the beta or the gamma subunits, results in increased ENaC activity. We report here that Nedd4-2 has two major isoforms that show tissue-specific expression; however, both isoforms can inhibit ENaC in Xenopus oocytes. Because there are four WW domains in Nedd4-2, we analyzed binding kinetics and affinity between individual WW domains and ENaC subunits. Using whole cell patch-clamp techniques, we studied the role of individual WW domains in the regulation of ENaC in mammalian cells. We report here that unlike Nedd4, only two of the Nedd4-2 WW domains, WW3 and WW4, are required for both the binding to ENaC subunits and the regulation of Na+ feedback control of ENaC. Although both WW3 and WW4 individually can interact with all three ENaC subunits in vitro, both domains together are essential for in vivo function of Nedd4-2 in ENaC regulation. These data suggest that Nedd4-2 WW3 and WW4 interact with distinct, noninterchangeable sites in ENaC and that to prevent Na+ feedback control of ENaC it is necessary to occlude both sites.

Developmental changes in the management of acid loads during preimplantation mouse development

Intracellular pH recovery in Quackenbush Swiss mouse preimplantation embryos following acid loading was investigated under conditions of H+-monocarboxylate cotransporter inactivity. Isoform-sensitive inhibitors of Na+-H+ exchange (NHE) were used to block the Na+-dependent component of the response. A biphasic dose-response curve for HOE-694 and N-methylisopropylamiloride (MIA) suggested that two isoforms (putatively NHE1 and NHE3) are active in the oocyte, 1-cell, and 2-cell stages. By the blastocyst stage, loss of one of the MIA-sensitive NHE activities (putatively NHE3) was observed in isolated inner cell masses, and an MIA-resistant component of the recovery was identified. The MIA-resistant component was inhibited by 2 mM amiloride and enhanced by external K+ and by 4,4'-diisothiocyanostilbene-2,2'-disulfonate, suggesting NHE4 activity. However, unlike NHE4 in other tissues, the MIA-resistant component did not transport Li+ in exchange for H+, and reverse transcription-polymerase chain reaction detected NHE4 mRNA in the oocyte but not in later stages. Trophoblast, whether in intact or collapsed blastocysts, did not show measurable NHE activity or MIA-sensitive activity during recovery from acid load. Both trophoblast and pluriblast manifested an H+ conductance in response to acid load. This H+ conductance was first detected at the 8-cell stage and was blocked by zinc in the isolated inner cell mass but not in trophoblast. No other effective inhibitors of its activity were found.

Regulation of the epithelial sodium channel by N4WBP5A, a novel Nedd4/Nedd4-2-interacting protein

The amiloride-sensitive epithelial sodium channel (ENaC) plays a critical role in fluid and electrolyte homeostasis and consists of alpha, beta, and gamma subunits. The carboxyl terminus of each ENaC subunit contains a PPXY motif that is believed to be important for interaction with the WW domains of the ubiquitin-protein ligases, Nedd4 and Nedd4-2. Disruption of this interaction, as in Liddle's syndrome where mutations delete or alter the PPXY motif of either the beta or gamma subunits, has been shown to result in increased ENaC activity and arterial hypertension. Here we present evidence that N4WBP5A, a novel Nedd4/Nedd4-2-binding protein, is a potential regulator of ENaC. In Xenopus laevis oocytes N4WBP5A increases surface expression of ENaC by reducing the rate of ENaC retrieval. We further demonstrate that N4WBP5A prevents sodium feedback inhibition of ENaC possibly by interfering with the xNedd4-2-mediated regulation of ENaC. As N4WBP5A binds Nedd4/Nedd4-2 via PPXY motif/WW domain interactions and appears to be associated with specific intracellular vesicles, we propose that N4WBP5A functions by regulating Nedd4/Nedd4-2 availability and trafficking. Because N4WBP5A is highly expressed in native renal collecting duct and other tissues that express ENaC, it is a likely candidate to modulate ENaC function in vivo.

Antisense co-suppression of G(alpha)(q) and G(alpha)(11) demonstrates that both isoforms mediate M(3)-receptor-activated Ca(2+) signalling in intact epithelial cells

We used replication-deficient adenoviruses overexpressing antisense against G(q) class alpha-subunits to determine the roles of G(q) and G(11) in mediating M(3)-receptor-coupled Ca(2+) mobilization in intact HT29 human colonic carcinoma epithelial cells. Western blot analysis and confocal microscopy showed that the viruses expressing antisense directed against the alpha-subunits of G(q) or G(11) produced isoform-specific reductions in the levels of these alpha-subunits. Fura-2 was used to measure changes in the Ca(2+) response following activation of the M(3) receptors by carbachol. The G(alpha)(q) antisense virus suppressed the peak Ca(2+) response by 70%, whereas the G(alpha)(11) antisense virus reduced it by 34%. We then used co-infection with both viruses to determine the effect of concomitant suppression of both G(alpha)(q) and G(alpha)(11). Overexpression of antisense to both alpha-subunits reduced by approximately 50% the levels of both G(alpha)(q) and G(alpha)(11). It also almost completely inhibited the Ca(2+) response to carbachol. These data show that both G(q) and G(11) are involved in mediating the action of the M(3) receptor on cytosolic Ca(2+) in HT29 cells. Furthermore, they suggest that the coupling of the M(3) receptor to these G proteins is specific, in that G(alpha)(q) cannot substitute for G(alpha)(11), and vice versa.

Intracellular Ca2+release by flufenamic acid and other blockers of the non-selective cation channel

We report in this paper using measurement of intracellular free Ca2+ with fura-2, that flufenamic acid and several related blockers of the 25 pS Ca(2+)-activated non-selective cation channel cause release of Ca2+ from an intracellular store other than the endoplasmic reticulum, possibly from mitochondria. A new compound, 4'-methyl-DPC, is found to be as effective in blocking non-selective cation channels as other flufenamate analogs but, like the parent compound, the non-selective cation channel blocker DPC, it does not cause release of Ca2+ from intracellular stores. DPC and 4'-methyl-DPC are thus the most suitable of the available blockers of non-selective cation channels for use in studies on the role of these channels in normal cell function.

Roles of the C termini of alpha -, beta -, and gamma -subunits of epithelial Na+ channels (ENaC) in regulating ENaC and mediating its inhibition by cytosolic Na+

The amiloride-sensitive epithelial Na(+) channels (ENaC) in the intralobular duct cells of mouse mandibular glands are inhibited by the ubiquitin-protein ligase, Nedd4, which is activated by increased intracellular Na(+). In this study we have used whole-cell patch clamp methods in mouse mandibular duct cells to investigate the role of the C termini of the alpha-, beta-, and gamma-subunits of ENaC in mediating this inhibition. We found that peptides corresponding to the C termini of the beta- and gamma-subunits, but not the alpha-subunit, inhibited the activity of the Na(+) channels. This mechanism did not involve Nedd4 and probably resulted from the exogenous C termini interfering competitively with the protein-protein interactions that keep the channels active. In the case of the C terminus of mouse beta-ENaC, the interacting motif included betaSer(631), betaAsp(632), and betaSer(633). In the C terminus of mouse gamma-ENaC, it included gammaSer(640). Once these motifs were deleted, we were able to use the C termini of beta- and gamma-ENaC to prevent Nedd4-mediated down-regulation of Na(+) channel activity. The C terminus of the alpha-subunit, on the contrary, did not prevent Nedd4-mediated inhibition of the Na(+) channels. We conclude that mouse Nedd4 interacts with the beta- and gamma-subunits of ENaC.

The Nedd4-like protein KIAA0439 is a potential regulator of the epithelial sodium channel

The amiloride-sensitive epithelial sodium channel (ENaC) plays a critical role in fluid and electrolyte homeostasis and consists of alpha, beta, and gamma subunits. The carboxyl terminus of each ENaC subunit contains a PPxY, motif which is believed to be important for interaction with the WW domains of the ubiquitin-protein ligase, Nedd4. Disruption of this interaction, as in Liddle's syndrome, where mutations delete or alter the PPxY motif of either the beta or gamma subunits, has been proposed to result in increased ENaC activity. Here we present evidence that KIAA0439 protein, a close relative of Nedd4, is also a potential regulator of ENaC. We demonstrate that KIAA0439 WW domains bind all three ENaC subunits. We show that a recombinant KIAA0439 WW domain protein acts as a dominant negative mutant that can interfere with the Na(+)-dependent feedback inhibition of ENaC in whole-cell patch clamp experiments. We propose that KIAA0439 and Nedd4 proteins either play a redundant role in ENaC regulation or function in a tissue- and/or signal-specific manner to down-regulate ENaC.

An antisense of protein kinase C-zeta inhibits proliferation of human airway smooth muscle cells

We hypothesized that an atypical isoform of protein kinase (PK) C, PKC-zeta, is essential for proliferation of human airway smooth muscle (HASM) cells in primary culture. Recombinant replication-deficient E1-deleted adenoviruses (100 plaque-forming units [pfu]/cell) expressing the antisense of PKC-zeta and the wild-type PKC-zeta (Ad-CMV-PKC-zeta) were added to actively growing cells that were subsequently incubated for 48 h in platelet-derived growth factor (PDGF) 40 ng/mL or 10% fetal bovine serum (FBS). Expression of the antisense at a virus concentration of 100 pfu/cell produced a significant (n = 3, P<0.05) decrease in the mean manual cell count in the presence of PDGF to 37+/-5% relative to that in cells with no virus (100%), whereas in cells infected with virus containing no construct, this figure was 102+/-13%. The increase in cell number in response to FBS, however, was not affected by the presence of the antisense. Corresponding values for cells in 10% FBS were 100+/-22%, 85+/-22%, and 122+/-18%. Western blotting revealed decreased levels of PKC-zeta protein, but not PKC-alpha or PKC-epsilon protein, in cells infected with the antisense when compared with levels in control cells. Thus, in HASM cells, PKC-zeta is involved in proliferation in response to PDGF, but not in response to FBS, for which alternate signal transduction pathways independent of PKC-zeta must exist.

The distribution of P2X receptor clusters on individual neurons in sympathetic ganglia and their redistribution on agonist activation

The distribution of P2X receptors on neurons in rat superior cervical ganglia and lability of P2X receptors on exposure to agonists were determined. Antibody labeling of each P2X subtype P2X(1)-P2X(7) showed neurons isolated into culture possessed primarily P2X(2) subunits with others occurring in order P2X(7) > P2X(6) > P2X(3) > P2X(1) > P2X(5) > P2X(4). Application of ATP and alpha,beta-meATP to neurons showed they possessed a predominantly nondesensitizing P2X receptor type insensitive to alpha,beta-meATP, consistent with immunohistochemical observations. P2X(1)-green fluorescent protein (GFP) was used to study the time course of P2X(1) receptor clustering in plasma membranes of neurons and internalization of receptors following prolonged exposure to ATP. At 12-24 h after adenoviral infection, P2X(1)-GFP formed clusters about 1 microm diameter in the neuron membrane. Application of ATP and alpha,beta-meATP showed these neurons possessed a predominantly desensitizing P2X receptor type sensitive to alpha,beta-meATP. Infection converted the major functional P2X receptor type in the membrane to P2X(1). Exposure of infected neurons to alpha,beta-meATP for less than 60 s led to the disappearance of P2X(1)-GFP fluorescence from the cell surface that was blocked by monensin, indicating the chimera is normally endocytosed into these organelles on exposure to agonist.

Influenza virus inhibits amiloride-sensitive Na+ channels in respiratory epithelia

Many pathogens causing diarrhea do so by modulating ion transport in the gut. Respiratory pathogens are similarly associated with disturbances of fluid balance in the respiratory tract, although it is not known whether they too act by altering epithelial ion transport. Here we show that influenza virus A/PR/8/34 inhibits the amiloride-sensitive Na(+) current across mouse tracheal epithelium with a half-time of about 60 min. We further show that the inhibitory effect of the influenza virus is caused by the binding of viral hemagglutinin to a cell-surface receptor, which then activates phospholipase C and protein kinase C. Given the importance of epithelial Na(+) channels in controlling the amount of fluid in the respiratory tract, we suggest that down-regulation of Na(+) channels induced by influenza virus may play a role in the fluid transport abnormalities that are associated with influenza infections.

P2X(1) receptor membrane redistribution and down-regulation visualized by using receptor-coupled green fluorescent protein chimeras

The P2X(1) purinergic receptor subtype occurs on smooth muscle cells of the vas deferens and urinary bladder where it is localized in two different size receptor clusters, with the larger beneath autonomic nerve terminal varicosities. We have sought to determine whether these synaptic-size clusters only form in the presence of varicosities and whether they are labile when exposed to agonists. P2X(1) and a chimera of P2X(1) and green fluorescent protein (GFP) were delivered into cells using microinjection, transient transfection or infection with a replication-deficient adenovirus. The P2X(1)-GFP chimera was used to study the time course of P2X(1) receptor clustering in plasma membranes and the internalization of the receptor following prolonged exposure to ATP. Both P2X(1) and P2X(1)-GFP clustered in the plasma membranes of Xenopus oocytes, forming patches 4-6 microm in diameter. Human embryonic kidney 293 (HEK293) cells, infected with the adenovirus, possessed P2X(1) antibody-labeled regions in the membrane colocalized with GFP fluorescence. The ED(50) for the binding of alpha,beta-methylene adenosine triphosphate (alpha,beta-meATP) to the P2X(1)-GFP chimera was similar to native P2X(1) receptors. ATP-generated whole-cell currents in oocytes or HEK293 cells expressing either P2X(1) or P2X(1)-GFP were similar. Exposure of HEK293 cells to alpha, beta-meATP for 10-20 min in the presence of 5 microM monensin led to the disappearance of P2X(1)-GFP fluorescence from the surface of the cells. These observations using the P2X(1)-GFP chimera demonstrate that P2X(1) receptors spontaneously form synaptic-size clusters in the plasma membrane that are internalized on exposure to agonists.

Studying heterotrimeric G-protein-linked signal transduction using replication-deficient adenoviruses

Plasma membrane-spanning G-protein-linked receptors transduce approximately 60% of all extracellular stimuli in higher animals. Many G-protein-linked receptor pathways are yet to be elucidated, with the receptor, G-protein or effector system as yet unidentified. In addition, many fundamental issues pertaining to G-protein signalling remain unresolved, such as the factors governing the specificity of G-protein receptor coupling and the control of signal amplitude in response to G-protein activation. In order to address some of these issues, the use of replication-deficient adenoviruses as gene transfer vectors for investigations of G-protein signalling has been developed, facilitating dissection of G-protein-linked signal transduction pathways in an extensive range of cultured cells, as well as in vivo. The present review focuses on the versatility and utility of adenoviruses for the investigation of signalling by heterotrimeric G-proteins and explores some of the recent advances in adenoviral technology as they relate to the study of signal transduction.

Characterization and functional significance of calcium transients in the 2-cell mouse embryo induced by an autocrine growth factor

Growth of preimplantation embryos is influenced by autocrine trophic factors that need to act by the 2-cell stage, but their mode of action is not yet described. This report shows that late zygote and 2-cell stage mouse embryos responded to embryo-derived platelet-activating factor (PAF) with transient increases in intracellular calcium concentration ([Ca(2+)](i)). [Ca(2+)](i) transients were single global events and were specifically induced by embryo-derived PAF. They were blocked by inhibition of phospholipase C (U 73122) and an inositol trisphosphate (IP(3)) receptor antagonist (xestospongin C), indicating the release of calcium from IP(3)-sensitive intracellular stores. Transients were also inhibited by the absence of calcium from extracellular medium and partially inhibited by treatment with dihydropyridine (nifedipine, 10 micrometer), but not pimozide (an inhibitor of an embryonic T-type calcium channel). (+/-)BAY K8644 (an L-type channel agonist) induced [Ca(2+)](i) transients, yet these were completely inhibited by nifedipine (10 micrometer). The complete inhibition of BAY K8644, but only partial inhibition of PAF by nifedipine shows that L-type channels were only partly responsible for the calcium influx. Depolarization of 2-cell embryos by 50 mm K(+) did not inhibit PAF-induced calcium transients, showing that the influx channels were not voltage-dependent. Depletion of intracellular calcium stores by thapsigargin revealed the presence of store-operated channels. The interdependent requirement for IP(3)-sensitive internal calcium stores and extracellular calcium in the generation of PAF-induced transients may be explained by a requirement for capacitative calcium entry via store-operated channels. A functionally important role for the PAF-induced transients is supported by the observation that inhibition of [Ca(2+)](i) transients by a PAF-antagonist (WEB 2086) or an intracellular calcium chelator (1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetrakis-acetoxymethyl ester; BAPTA-AM) caused marked inhibition of early embryo development. Growth inhibition by BAPTA-AM was relieved by addition of exogenous PAF.

Purinergic responses in HT29 colonic epithelial cells are mediated by G protein alpha -subunits

Using Fura-2 to measure changes in intracellular calcium ([Ca(2+)](i)), we show that P(2U)receptors in HT29 cells trigger an increase in [Ca(2+)](i)by pertussis toxin-insensitive G proteins. We then use replication-deficient adenoviruses expressing wild-type and dominant negative mutants of G(alpha q)and G(alpha i2), antisense directed against G(alpha q)or the C-terminal fragment of beta-adrenergic receptor kinase (beta ARK-CT) to identify these G proteins. We find the [Ca(2+)](i)response to UTP is not affected by increased expression of the wild-type G(alpha q), wild-type G(alpha i2)or beta ARK-CT, while it is blocked by over-expression of dominant negative G(alpha q). The timecourse of the UTP response is, however, altered by wild-type G(alpha q)and is only weakly inhibited by antisense G(alpha q). This suggests that the P(2U)response is mediated, at least partially, by a G protein distinct from G(alpha q). In contrast, the M(3)muscarinic response is inhibited by over-expression of antisense against G(alpha q), or over-expression of beta ARK-CT, a finding in agreement with our previous observation that the muscarinic response in HT29 cells is mediated by the beta gamma-subunits of G(q). We also find that P(2U)and M(3)receptors do not control identical Ca(2+)stores, suggesting that differential activation of G proteins can lead to Ca(2+)release from distinct stores.