Modulation of epithelial sodium channel trafficking and function by sodium 4-phenylbutyrate in human nasal epithelial cells

Sodium 4-phenylbutyrate (4-PBA) has been shown to correct the cellular trafficking of several mutant or nonmutant plasma membrane proteins such as cystic fibrosis transmembrane conductance regulator through the expression of 70-kDa heat shock proteins. The objective of the study was to determine whether 4-PBA may influence the functional expression of epithelial sodium channels (ENaC) in human nasal epithelial cells (HNEC). Using primary cultures of HNEC, we demonstrate that 4-PBA (5 mm for 6 h) markedly stimulated amiloride-sensitive sodium channel activity and that this was related to an increased abundance of alpha-, beta-, and gamma-ENaC subunits in the apical membrane. The increase in ENaC cell surface expression (i) was due to insertion of newly ENaC subunits as determined by brefeldin A experiments and (ii) was not associated with cell surface retention of ENaC subunits because endocytosis of ENaC subunits was unchanged. In addition, we find that ENaC co-immunoprecipitated with the heat shock protein constitutively expressed Hsc70, that has been reported to modulate ENaC trafficking, and that 4-PBA decreased Hsc70 protein level. Finally, we report that in cystic fibrosis HNEC obtained from two cystic fibrosis patients, 4-PBA increased functional expression of ENaC as demonstrated by the increase in amiloride-sensitive sodium transport and in alpha-, beta-, and gamma-ENaC subunit expression in the apical membrane. Our results suggest that in HNEC, 4-PBA increases the functional expression of ENaC through the insertion of new alpha-, beta-, and gamma-ENaC subunits into the apical membrane and also suggest that 4-PBA could modify ENaC trafficking by reducing Hsc70 protein expression.     

K+ channel cAMP activated in guinea pig gallbladder epithelial cells

In guinea pig gallbladder epithelial cells, an increase in intracellular cAMP levels elicits the rise of anion channel activity. We investigated by patch-clamp techniques whether K(+) channels were also activated. In a cell-attached configuration and in the presence of theophylline and forskolin or 8-Br-cAMP in the cellular incubation bath, an increase of the open probability (P(o)) values for Ca(2+)-activated K(+) channels with a single-channel conductance of about 160 pS, for inward current, was observed. The increase in P(o) of these channels was also seen in an inside-out configuration and in the presence of PKA, ATP, and cAMP, but not with cAMP alone; phosphorylation did not influence single-channel conductance. In the inside-out configuration, the opioid loperamide (10(-5) M) was able to reduce P(o) when it was present either in the microelectrode filling solution or on the cytoplasmic side. Detection in the epithelial cells by RT-PCR of the mRNA corresponding to the alpha subunit of large-conductance Ca(2+)-activated K(+) channels (BK(Ca)) indicates that this gallbladder channel could belong to the BK family. Immunohistochemistry experiments confirm that these cells express the BK alpha subunit, which is located on the apical membrane. Other K(+) channels with lower conductance (40 pS) were not activated either by 8-Br-cAMP (cell-attached) or by PKA + ATP + cAMP (inside-out). These channels were insensitive to TEA(+) and loperamide. The data demonstrate that under conditions that induce secretion, phosphorylation activates anion channels as well as Ca(2+)-dependent, loperamide-sensitive K(+) channels present on the apical membrane.     

Activation of NF-kappaB in airway epithelial cells is dependent on CFTR trafficking and Cl- channel function

Polymorphonuclear leukocyte-dominated airway inflammation is a major component of cystic fibrosis (CF) lung disease and may be associated with CF transmembrane conductance regulator (CFTR) dysfunction as well as infection. Mutant DeltaF508 CFTR is mistrafficked, accumulates in the endoplasmic reticulum (ER), and may cause "cell stress" and activation of nuclear factor (NF)-kappaB. G551D mutants also lack Cl- channel function, but CFTR is trafficked normally. We compared the effects of CFTR mutations on the endogenous activation of an NF-kappaB reporter construct. In transfected Chinese hamster ovary cells, the mistrafficked DeltaF508 allele caused a sevenfold activation of NF-kappaB compared with wild-type CFTR or the G551D mutant (P < 0.001). NF-kappaB was also activated in 9/HTEo-/pCep-R cells and in 16HBE/pcftr antisense cell lines, which lack CFTR Cl- channel function but do not accumulate mutant protein in the ER. This endogenous activation of NF-kappaB was associated with elevated interleukin-8 expression. Impaired CFTR Cl- channel activity as well as cell stress due to accumulation of mistrafficked CFTR in the ER contributes to the endogenous activation of NF-kappaB in cells with the CFTR mutation.     

K+ transport in 'tight' epithelial monolayers of MDCK cells. Evidence for a calcium-activated K+ channel

Measurements of 86Rb efflux across the apical and basal-lateral aspects of intact monolayers of 'high-resistance' MDCK cells mounted in Ussing chambers have been made. A transient increase in 86Rb efflux across both epithelial borders upon stimulation with adrenalineeeeeee or ionophore A23187 is observed. The increased 86Rb across the basal cell aspects is of greatest quantitative importance. Measurements of total cellular K+ contents by flame photometry of tissue extracts indicate a net loss of K+ following adrenalin addition. The effects of adrenalin and ionophore A23187 upon 86Rb efflux are abolished in 'Ca2+ -free' media. The properties of the Ca2+ -dependent increase in 86Rb efflux show similarities to Ca2+ -activated K+ conductances in other tissues, notably human red cells, including inhibition by quinine (1 mM), tetraethylammonium (25 mM) and insensitivity to bee venom toxin (apamin) (25 nM). Adrenalin is only effective when applied to the basal bathing solution suggesting that the receptors mediating adrenalin action are located upon the basal-lateral membranes. Half maximal stimulation of 86Rb efflux by adrenalin is observed at 9.1 X 10(-7) M. The action of various adrenergic receptor agonists and antagonists are consistent with adrenalin action being mediated by an alpha-adrenergic receptor.     

RPTP{micro} and protein tyrosine phosphorylation regulate K+ channel mRNA expression in adult cardiac myocytes

Previously, we reported that cell-cell contact regulatesK⁺ channel mRNA expression in cultured adult rat cardiacmyocytes. Here we show that exposing cardiac myocytes to tyrosinekinase inhibitors (genistein, tyrphostin A25), but not inactiveanalogs, prevents downregulation of Kv1.5 mRNA and upregulation ofKv4.2 mRNA normally observed when they are cultured under low-density conditions. Furthermore, cardiac myocytes cocultured with cells thatendogenously (Mv 1 Lu) or heterologously (Chinese hamster ovary cells)express the receptor-type protein tyrosine phosphatase µ (RPTPµ)display Kv1.5 mRNA levels paralleling that which was observed inmyocytes cultured under high-density conditions and in intact tissue.In contrast, myocytes cocultured with control cells failed to producethis response. Finally, it is shown that Kv4.2 mRNA expression isunaffected by RPTPµ. These findings reveal that multiple tyrosinephosphorylation-dependent mechanisms control cardiac myocyteK⁺ channel genes. Furthermore, we conclude that RPTPµspecifically regulates cardiac myocyte Kv1.5 mRNA expression. Thus thisreceptor protein tyrosine phosphatase may be important in responses to pathological conditions associated with the loss of cell-cell interactions in the heart.

     

Developmental analysis reveals mismatches in the expression of K+ channel alpha subunits and voltage-gated K+ channel currents in rat ventricular myocytes

In the experiments here, the developmental expression of the functional Ca(2+)-independent, depolarization-activated K+ channel currents, Ito and IK, and of the voltage-gated K+ channel (Kv) alpha subunits, Kv1.2, Kv1.4, Kv1.5, Kv2.1, and Kv4.2 in rat ventricular myocytes were examined quantitatively. Using the whole-cell patch clamp recording method, the properties and the densities of Ito and IK in ventricular myocytes isolated from postnatal day 5 (P5), 10 (P10), 15 (P15), 20 (P20), 25 (P25), 30 (P30), and adult (8-12 wk) rats were characterized and compared. These experiments revealed that mean Ito densities increase fourfold between birth and P30, whereas IK densities vary only slightly. Neither the time- nor the voltage-dependent properties of the currents vary measurably, suggesting that the subunits underlying functional Ito and IK channels are the same throughout postnatal development. In parallel experiments, the developmental expression of each of the voltage-gated K+ channel alpha subunits, Kv1.2, Kv1.4, Kv1.5, Kv2.1, and Kv4.2, was examined quantitatively at the mRNA and protein levels using subunit-specific probes. RNase protection assays revealed that Kv1.4 message levels are high at birth, increase between P0 and P10, and subsequently decrease to very low levels in adult rat ventricles. The decrease in message is accompanied by a marked reduction in Kv1.4 protein, consistent with our previous suggestion that Kv1.4 does not contribute to the formation of functional K+ channels in adult rat ventricular myocytes. In contrast to Kv1.4, the mRNA levels of Kv1.2, Kv1.5, Kv2.1, and Kv4.2 increase (three- to five- fold) between birth and adult. Western analyses, however, revealed that the expression patterns of these subunits proteins vary in distinct ways: Kv1.2 and Kv4.2, for example, increase between P5 and adult, whereas Kv1.5 remains constant and Kv2.1 decreases. Throughout development, therefore, there is a mismatch between the numbers of Kv alpha subunits expressed and the functional voltage-gated K+ channel currents distinguished electrophysiologically in rat ventricular myocytes. Alternative experimental approaches will be required to define directly the Kv alpha subunits that underlie functional voltage- gated K+ channels in these (and other) cells. In addition, the finding that Kv alpha subunit protein expression levels do not necessarily mirror mRNA levels suggests that caution should be exercised in attempting functional interpretations of observed changes in mRNA levels alone.     

Cell-cell contact between adult rat cardiac myocytes regulates Kv1.5 and Kv4.2 K+ channel mRNA expression

Regulation of voltage-gatedK⁺ channel genes represents animportant mechanism for modulating cardiac excitability. Here we demonstrate that expression of twoK⁺ channel mRNAs is reciprocallycontrolled by cell-cell interactions between adult cardiac myocytes. Itis shown that culturing acutely dissociated rat ventricular myocytesfor 3 h results in a dramatic downregulation of Kv1.5 mRNA and a modestupregulation of Kv4.2 mRNA. These effects are specific, because similarchanges are not detected with other channel mRNAs. Increasing myocytedensity promotes maintenance of Kv1.5 gene expression, whereas Kv4.2mRNA expression was found to be inversely proportional to cell density. Conditioned culture medium did not mimic the effects of high cell density. However, paraformaldehyde-fixed myocytes were comparable tolive cells in their ability to influenceK⁺ channel message levels. Thusthe reciprocal effects of cell density on the expression of Kv1.5 andKv4.2 genes are mediated by direct contact between adult cardiacmyocytes. These findings reveal for the first time that cardiac myocytegene expression is influenced by signaling induced by cell-cell contact.

     

The potassium channel opener cromakalim (BRL 34915) activates ATP-dependent K+ channels in isolated cardiac myocytes

In cardiac myocytes, cromakalim (BRL 34915), a potassium channel opener, activates a time-independent K+ current exhibiting poor voltage-sensitivity. This effect of cromakalim is antagonized by low concentrations of glibenclamide, a specific blocker of ATP-dependent K+ channels in cardiac cells. Direct recording of the activity of K+ channels in inside-out membrane patches, confirmed that cromakalim is a potent activator of ATP-dependent K+ channels in cardiac myocytes.     

K+ transport in ‘tight’ epithelial monolayers of MDCK cells: Evidence for a calcium-activated K+ channel

Measurements of ⁸⁶Rb efflux across the apical and basal-lateral aspects of intact monolayers of ‘high-resistance’ MDCK cells mounted in Ussing chambers have been made. A transient increase in ⁸⁶Rb efflux across both epithelial borders upon stimulation with adrenalin or ionophore A23187 is observed. The increased ⁸⁶Rb across the basal cell aspects is of greatest quantitative importance. Measurements of total cellular K⁺ contents by flame photometry of tissue extracts indicate a net loss of K⁺ following adrenalin addition. The effects of adrenalin and ionophore A23187 upon ⁸⁶Rb efflux are abolished in ‘Ca²⁺-free’ media. The properties of the Ca²⁺ -dependent increase in ⁸⁶Rb efflux show similarities to Ca²⁺-activated K⁺ conductances in other tissues, notably human red cells, including inhibition by quinine (1 mM), tetraethylammonium (25 mM) and insensitivity to bee venom toxin (apamin) (25 nM). Adrenalin is only effective when applied to the basal bathing solution suggesting that the receptors mediating adrenalin action are located upon the basal-lateral membranes. Half maximal stimulation of ⁸⁶Rb efflux by adrenalin is observed at 9.1·10^?7 M. The action of various adrenergic receptor agonists and antagonists are consistent with adrenalin action being mediated by an α-adrenergic receptor.     

Whole-cell and single channel K+ and Cl- currents in epithelial cells of frog skin.

Whole-cell and single channel currents were studied in cells from frog (R. pipiens and R. catesbiana) skin epithelium, isolated by collagenase and trypsin treatment, and kept in primary cultures up to three days. Whole-cell currents did not exhibit any significant time-dependent kinetics under any ionic conditions used. With an external K gluconate Ringer solution the currents showed slight inward rectification with a reversal potential near zero and an average conductance of 5 nS at reversal. Ionic substitution of the external medium showed that most of the cell conductance was due to K and that very little, if any, Na conductance was present. This confirmed that most cells originate from inner epithelial layers and contain membranes with basolateral properties. At voltages more positive than 20 mV outward currents were larger with K in the medium than with Na or N-methyl-D-glucamine. Such behavior is indicative of a multi-ion transport mechanism. Whole-cell K current was inhibited by external Ba and quinidine. Blockade by Ba was strongly voltage dependent, while that by quinidine was not. In the presence of high external Cl, a component of outward current that was inhibited by the anion channel blocker diphenylamine-2-carboxylate (DPC) appeared in 70% of the cells. This component was strongly outwardly rectifying and reversed at a potential expected for a Cl current. At the single channel level the event most frequently observed in the cell-attached configuration was a K channel with the following characteristics: inward-rectifying I-V relation with a conductance (with 112.5 mM K in the pipette) of 44 pS at the reversal potential, one open and at least two closed states, and open probability that increased with depolarization. Quinidine blocked by binding in the open state and decreasing mean open time. Several observations suggest that this channel is responsible for most of the whole-cell current observed in high external K, and for the K conductance of the basolateral membrane of the intact epithelium. On a few occasions a Cl channel was observed that activated upon excision and brief strong depolarization. The I-V relation exhibited strong outward rectification with a single channel conductance of 48 pS at 0 mV in symmetrical 112 mM Cl solutions. Kinetic analysis showed the presence of two open and at least two closed states. Open time constants and open probability increased markedly with depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)     

Mechanism of K+ channel block by verapamil and related compounds in rat alveolar epithelial cells

The mechanism by which the phenylalkylamines, verapamil and D600, and related compounds, block inactivating delayed rectifier K+ currents in rat alveolar epithelial cells, was investigated using whole-cell tight- seal recording. Block by phenylalkylamines added to the bath resembles state-dependent block of squid K+ channels by internally applied quarternary ammonium ions (Armstrong, C.M. 1971. Journal of General Physiology. 58:413-437): open channels are blocked preferentially, increased [K+]o accelerates recovery from block, and recovery occurs mainly through the open state. Slow recovery from block is attributed to the existence of a blocked-inactivated state, because recovery was faster in three situations where recovery from inactivation is faster: (a) at high [K+]o, (b) at more negative potentials, and (c) in cells with type l K+ channels, which recover rapidly from inactivation. The block rate was used as a bioassay to reveal the effective concentration of drug at the block site. When external pH, pHo, was varied, block was much faster at pHo 10 than pHo 7.4, and very slow at pHo 4.5. The block rate was directly proportional to the concentration of neutral drug in the bath, suggesting that externally applied drug must enter the membrane in neutral form to reach the block site. High internal pH (pHi 10) reduced the apparent potency of externally applied phenylalkylamines, suggesting that the cationic form of these drugs blocks K+ channels at an internal site. The permanently charged analogue D890 blocked more potently when added to the pipette than to the bath. However, lowering pHi to 5.5 did not enhance block by external drug, and tertiary phenylalkylamines added to the pipette solution blocked weakly. This result can be explained if drug diffuses out of the cell faster than it is delivered from the pipette, the block site is reached preferentially via hydrophobic pathways, or both. Together, the data indicate the neutral membrane-bound drug blocks K+ channels more potently than intracellular cationic drug. Neutral drug has rapid access to the receptor, where block is stabilized by protonation of the drug from the internal solution. In summary, externally applied phenylalkylamines block open or inactivated K+ channels by partitioning into the cell membrane in neutral form and are stabilized at the block site by protonation.     

Effects of ADP upon the ATP-sensitive K+ channel in rat ventricular myocytes

Summary The effects of ADP upon the gating of ATP-sensitive K⁺ channels from rat ventricular myocytes have been investigated by patch-clamp single-channel current recording experiments. ADP was applied to the internal surface of excised insideout membrane patches and depending upon the experimental protocol and the concentration it was found that ADP could either inhibit or stimulate openings of ATP-sensitive K⁺ channels. In the absence of inactivation, ATP-sensitive K⁺ channels were inhibited by ADP in a dose-dependent manner. Partially inactivated channels, on the other hand, were stimulated by low (10 to 250 M) and inhibited by high (>250 M) concentrations of ADP. ATP-sensitive K⁺ channels which were being inhibited by ATP (<1 mM) could be opened by the simultaneous application of ADP (50 M to 1 mM). ADP had no effect upon channels inhibited by mM concentrations of ATP. The situation was further complicated when it was found that inhibition evoked by ADP was strongly attenuated by the presence of Mg²⁺ ions whilst channel stimulation, whether of partially inactivated channels or channels inhibited by ATP, required the presence of Mg²⁺ ions. The analog of ADP, ADPS, always evoked inhibition of ATP-sensitive K⁺ channels which was not affected by the presence or absence of Mg²⁺ ions.     

Chloride and potassium channel function in alveolar epithelial cells

Electrolyte transport across the adult alveolar epithelium plays an important role in maintaining a thin fluid layer along the apical surface of the alveolus that facilitates gas exchange across the epithelium. Most of the work published on the transport properties of alveolar epithelial cells has focused on the mechanisms and regulation of Na(+) transport and, in particular, the role of amiloride-sensitive Na(+) channels in the apical membrane and the Na(+)-K(+)-ATPase located in the basolateral membrane. Less is known about the identity and role of Cl(-) and K(+) channels in alveolar epithelial cells, but studies are revealing important functions for these channels in regulation of alveolar fluid volume and ionic composition. The purpose of this review is to examine previous work published on Cl(-) and K(+) channels in alveolar epithelial cells and to discuss the conclusions and speculations regarding their role in alveolar cell transport function.     

Chronic exposure to EGF affects trafficking and function of ENaC channel in cystic fibrosis cells

Using the whole-cell patch-clamp technique, we identified an amiloride (AMI)-sensitive Na(+) current in cystic fibrosis cells, JME/CF15, growing in standard medium. The reversal potential of this current depended on Na(+) concentrations and the cation selectivity was much higher for Na(+) than for K(+), indicating that the current is through ENaC channels. In contrast, cells from EGF-containing medium lacked AMI-sensitive Na(+) currents. In permeabilized cells growing in EGF-containing medium, alphaENaC was mainly detected in a perinuclear region, while in cells from standard medium it was distributed over the cell body. Western-blot analysis showed that in standard medium cells expressed fast-migrating EndoH-insensitive and slow-migrating EndoH-sensitive alphaENaC fractions, while in cells growing in the presence of EGF, alphaENaC was only detected as the fast-migrating EndoH-insensitive fraction. Long-term incubation of cells with EGF resulted in an increased basal Ca(2+) level, [Ca(2+)](i). A similar increase of [Ca(2+)](i) was also observed in the presence of 2muM thapsigargin, resulting in inhibition of ENaC function. Thus, in JME/CF15 cells inhibition of the ENaC function by chronic incubation with EGF is a Ca(2+)-mediated process that affects trafficking and surface expression of ENaC channels.     

PKC regulation of cardiac CFTR Cl- channel function in guinea pig ventricular myocytes

The role ofprotein kinase C (PKC) in regulating the protein kinase A(PKA)-activated Cl currentconducted by the cardiac isoform of the cystic fibrosis transmembraneconductance regulator (cCFTR) was studied in guinea pig ventricularmyocytes using the whole cell patch-clamp technique. Althoughstimulation of endogenous PKC with phorbol 12,13-dibutyrate (PDBu)alone did not activate thisCl current, even whenintracellular dialysis was limited with the perforated patch-clamptechnique, activation of PKC did elicit a significant response in thepresence of PKA-dependent activation of the current by the-adrenergic receptor agonist isoproterenol. PDBuincreased the magnitude of theCl conductance activated bya supramaximally stimulating concentration of isoproterenol by 21 ± 3.3% (n = 9) when added afterisoproterenol and by 36 ± 16% (n = 14) when introduced before isoproterenol. 4-Phorbol12,13-didecanoate, a phorbol ester that does not activate PKC, did notmimic these effects. Preexposure to chelerythrine orbisindolylmaleimide, two highly selective inhibitors of PKC, significantly reduced the magnitude of the isoproterenol-activated Cl current by 79 ± 7.7% (n = 11) and 52 ± 10%(n = 8), respectively. Ourresults suggest that although acute activation of endogenous PKC alonedoes not significantly regulate cCFTRCl channel activity innative myocytes, it does potentiate PKA-dependent responses, perhapsmost dramatically demonstrated by basal PKC activity, which may play apivotal role in modulating the function of these channels.