Changes in the expression of the Alzheimer’s disease-associated presenilin gene in drosophila heart leads to cardiac dysfunction

Mutations in the presenilin genes cause the majority of early-onset familial Alzheimer’s disease. Recently, presenilin mutations have been identified in patients with dilated cardiomyopathy (DCM), a common cause of heart failure and the most prevalent diagnosis in cardiac transplantation patients. However, the molecular mechanisms, by which presenilin mutations lead to either AD or DCM, are not yet understood. We have employed transgenic Drosophila models and optical coherence tomography imaging technology to analyze cardiac function in live adult Drosophila. Silencing of Drosophila ortholog of presenilins (dPsn) led to significantly reduced heart rate and remarkably age-dependent increase in end-diastolic vertical dimensions. In contrast, overexpression of dPsn increased heart rate. Either overexpression or silencing of dPsn resulted in irregular heartbeat rhythms accompanied by cardiomyofibril defects and mitochondrial impairment. The calcium channel receptor activities in cardiac cells were quantitatively determined via real-time RT-PCR. Silencing of dPsn elevated dIP3R expression, and reduced dSERCA expression; overexprerssion of dPsn led to reduced dRyR expression. Moreover, overexpression of dPsn in wing disc resulted in loss of wing phenotype and reduced expression of wingless. Our data provide novel evidence that changes in presenilin level leads to cardiac dysfunction, owing to aberrant calcium channel receptor activities and disrupted Wnt signaling transduction, indicating a pathogenic role for presenilin mutations in DCM pathogenesis.

Morphological and electrical properties of human trophoblast choriocarcinoma, BeWo cells

The syncytiotrophoblast of the human placenta arises from fusion of stem cells called cytotrophoblasts. The molecular mechanisms associated with cell fusion and syncytiation of cytotrophoblastic cells remain largely unknown. In the present study, we investigated the morphological and electrical properties of BeWo cells, a human choriocarcinoma-derived trophoblast cell model, with several features of the human cytotrophoblast. Cultured cells tended to cluster, but only fused into small, multinucleated syncytia in the presence of cAMP (72 h). The morphological features of both the actin and microtubular cytoskeletons indicated that within 72 h of constant exposure to cAMP, intracellular cortical actin cytoskeleton disappeared, which was the most prominent inducing factor of multi-nucleation. The presence of the cation channel protein, polycystin-2 (PC2), a TRP-type cation channel, associated with placental ion transport in term human syncytiotrophoblast, co-localised with acetylated tubulin in midbodies, but was found non-functional under any conditions. Different electrical phenotypes were observed among control BeWo cells, where only 26% (8 of 31 cells) displayed a voltage-dependent outwardly rectifying conductance. Most quiescent BeWo cells had, however, a low, slightly outwardly rectifying basal whole cell conductance. Acute exposure to intracellular cAMP (<15 min) increased the whole cell conductance by 122%, from 0.72 nS/cell to 1.60 nS/cell, and eliminated the voltage-regulated conductance. The encompassed evidence indicates that the early events in BeWo cell fusion and syncytiation occur by cAMP-associated changes in ionic conductance but not morphological changes associated to chronic exposure to the second messenger. This suggests a tight regulation, and important contribution of cation conductances in cytotrophoblastic cells prior to syncytiation.

Reactive oxygen species inhibit polycystin-2 (TRPP2) cation channel activity in term human syncytiotrophoblast

Pregnancy is often associated with oxidative stress (OS) and lower antioxidant defences, which are both implicated in the pathophysiology of preeclampsia, free radical-induced birth defects, and abortions, as well as gestational diabetes mellitus (GDM), where products of lipid peroxidation are increased. The molecular target(s) of increased oxygen free radicals and consequent lipid peroxidation in the human placenta remains ill defined. The human syncytiotrophoblast (hST) expresses abundant polycystin-2 (PC2, TRPP2), a TRP-type Ca(2+)-permeable non-selective cation channel. Here, we explored the effect of reactive oxygen species (ROS) on PC2 channel activity of term hST. Apical membranes of the hST were reconstituted in a lipid bilayer chamber. Addition of either hydrogen-peroxide (H(2)O(2)) or tert-butyl hydroperoxide (tBHP) to the cis chamber (intracellular side) rapidly and completely inhibited PC2-mediated cation channel activity in reconstituted hST vesicles. A dose-response titration with increasing concentrations of H(2)O(2) gave an IC(50)=131 nM. The effect of H(2)O(2) on the isolated protein from in vitro transcribed/translated material was significantly different. H(2)O(2) inhibited PC2 cation channel activity, with a much lower affinity (IC(50)=193 microM). To correlate these findings with H(2)O(2)-induced lipid peroxidation, TBARS where measured in hST apical membranes incubated with H(2)O(2). Increased TBARS by exposure of hST apical membranes to H(2)O(2) (625 microM) returned to control value in the presence of catalase (167 microg/ml). Taken together these data indicate that ROS affect PC2 channel function by targetting both membrane lipids and the channel protein. Thus, OS in human pregnancy may be linked to dysregulation of channels such as PC2, which allow the transport of Ca(2+) into the placenta. Oxidative complications in pregnancy may implicate dysfunctional cation transfer between mother and fetus.

Co-inheritance of a PKD1 mutation and homozygous PKD2 variant: a potential modifier in autosomal dominant polycystic kidney disease

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD), which is caused by mutations in polycystins 1 (PC1) and 2 (PC2), is one of the most commonly inherited renal diseases, affecting ~1 : 1000 Caucasians.

MATERIALS AND METHODS

We screened Greek ADPKD patients with the denaturing gradient gel electrophoresis (DGGE) assay and direct sequencing.

RESULTS

We identified a patient homozygous for a nucleotide change c.1445T > G, resulting in a novel homozygous substitution of the non-polar hydrophobic phenylalanine to the polar hydrophilic cysteine in exon 6 at codon 482 (p.F482C) of the PKD2 gene and a de-novo PKD1 splice-site variant IVS21-2delAG. We did not find this PKD2 variant in a screen of 280 chromosomes of healthy subjects, supporting its pathogenicity. The proband's parents did not have the PKD1 mutation. Real-time PCR of the PKD2 transcript from a skin biopsy revealed 20-fold higher expression in the patient than in a healthy subject and was higher in the patient's peripheral blood mononuclear cells (PBMCs) than in those of her heterozygote daughter and a healthy subject. The greater gene expression was also supported by Western blotting. Inner medullar collecting duct (IMCD) cells transfected with the mutant PKD2 mouse gene presented a perinuclear and diffuse cytoplasmic localization compared with the wild type ER localization. Patch-clamping of PBMCs from the p.F482C homozygous and heterozygous subjects revealed lower polycystin-2 channel function than in controls.

CONCLUSIONS

We report for the first time a patient with ADPKD who is heterozygous for a de novo PKD1 variant and homozygous for a novel PKD2 mutation.

Ionic wave propagation along actin filaments

We investigate the conditions enabling actin filaments to act as electrical transmission lines for ion flows along their lengths. We propose a model in which each actin monomer is an electric element with a capacitive, inductive, and resistive property due to the molecular structure of the actin filament and viscosity of the solution. Based on Kirchhoff's laws taken in the continuum limit, a nonlinear partial differential equation is derived for the propagation of ionic waves. We solve this equation in two different regimes. In the first, the maximum propagation velocity wave is found in terms of Jacobi elliptic functions. In the general case, we analyze the equation in terms of Fisher-Kolmogoroff modes with both localized and extended wave characteristics. We propose a new signaling mechanism in the cell, especially in neurons.

Role of actin filament organization in CFTR activation

The cystic fibrosis transmembrane conductance regulator (CFTR) is an anion-selective channel whose dysfunction leads to the onset of cystic fibrosis. CFTR activation is normally elicited by stimulation of the cAMP pathway, which effects protein kinase A activation. However, previous studies from our laboratory indicate that the actin cytoskeleton is also required for a proper CFTR function. In this report, the regulatory role of actin filament organization in the activation of CFTR was explored. Maneuvers to modify the steady-state organization of actin filaments elicit the activation of CFTR in the absence of a functional cAMP pathway. Partial disruption of the actin cytoskeleton of CFTR-expressing cells with cytochalasin D (CD) induced CFTR activation in the absence of an activated PKA. Similar findings were obtained by intracellular dialysis with the actin-severing protein gelsolin. However, extended treatment with CD leading to the collapse of the actin cytoskeleton rendered CFTR completely insensitive to direct PKA activation. cAMP activation of CFTR was also found to be dysfunctional in cells lacking the actin-crosslinking protein ABP-280, which was recovered after dialysis of the cells with filamin, a homologue of ABP-280. The present data indicate that an organized actin network is required for the proper cAMP-dependent activation of CFTR. The possibility is also explored that actin must be directly associated with CFTR to elicit its activation, further suggesting that this channel protein may bind actin as well.

Electrodiffusional ATP movement through CFTR and other ABC transporters

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the superfamily of ATP-binding cassette (ABC) transporters, also known as traffic ATPases. Recent studies from our laboratory determined that various members of the ABC family of transport proteins mediate the electrodiffusional movement of the nucleotide ATP. In this report, evidence for the movement of cellular nucleotides by the ABC transporter CFTR and related molecules, including P-glycoproteins (Pgp), is reviewed. The wild-type mdr1 gene product, Pgp, enables the spontaneous release of cellular ATP. However, single amino acid substitutions in both nucleotide-binding sites render a dysfunctional Pgp, whose function can only be reversed by voltage activation. This report includes data indicating that reconstitution of highly purified CFTR from human epithelial origin enables the permeation of both Cl and ATP. The relevance of the ABC domains in ATP transport is also explored, and the hypothesis is forwarded that improper ATP transport by a dysfunctional CFTR is a relevant factor in cystic fibrosis.

Growth and function of the embryonic heart depend upon the cardiac-specific L-type calcium channel alpha1 subunit

The heart must function from the moment of its embryonic assembly, but the molecular underpinnings of the first heart beat are not known, nor whether function determines form at this early stage. Here, we find by positional cloning that the embryonic lethal island beat (isl) mutation in zebrafish disrupts the alpha1 C L-type calcium channel subunit (C-LTCC). The isl atrium is relatively normal in size, and individual cells contract chaotically, in a pattern resembling atrial fibrillation. The ventricle is completely silent. Unlike another mutation with a silent ventricle, isl fails to acquire the normal number of myocytes. Thus, calcium signaling via C-LTCC can regulate heart growth independently of contraction, and plays distinctive roles in fashioning both form and function of the two developing chambers.

Polycystin-2, the protein mutated in autosomal dominant polycystic kidney disease (ADPKD), is a Ca2+-permeable nonselective cation channel

Defects in polycystin-2, a ubiquitous transmembrane glycoprotein of unknown function, is a major cause of autosomal dominant polycystic kidney disease (ADPKD), whose manifestation entails the development of fluid-filled cysts in target organs. Here, we demonstrate that polycystin-2 is present in term human syncytiotrophoblast, where it behaves as a nonselective cation channel. Lipid bilayer reconstitution of polycystin-2-positive human syncytiotrophoblast apical membranes displayed a nonselective cation channel with multiple subconductance states, and a high perm-selectivity to Ca2+. This channel was inhibited by anti-polycystin-2 antibody, Ca2+, La3+, Gd3+, and the diuretic amiloride. Channel function by polycystin-2 was confirmed by patch-clamping experiments of polycystin-2 heterologously infected Sf9 insect cells. Further, purified insect cell-derived recombinant polycystin-2 and in vitro translated human polycystin-2 had similar ion channel activity. The polycystin-2 channel may be associated with fluid accumulation and/or ion transport regulation in target epithelia, including placenta. Dysregulation of this channel provides a mechanism for the onset and progression of ADPKD.

Increased circulating levels of plasma ATP in cystic fibrosis patients

Recent studies have shown that the cystic fibrosis transmembrane conductance regulator (CFTR), an ATP-binding cassette (ABC) transporter whose mutations are responsible for cystic fibrosis (CF), permeates ATP. However, little information is available concerning extracellular ATP concentrations in CF patients. Thus, the goal of this preliminary study was to determine the circulating levels of plasma ATP in CF patients. Circulating levels of plasma ATP were determined by the luciferin-luciferase assay in both CF patients and healthy volunteer control subjects. The two groups were compared using an analysis of variance. CF genotype and age, which ranged from 7 to 56 years, were also used to compare data by single-blind analysis. With comparable sample numbers, CF patients had statistically higher levels of circulating ATP (34%, P<0.01) when compared by analysis of covariance with the age of the subjects as the cofactor. The CF patients bearing the DeltaF508 genotype had a 54% (n=33, P<0.01) higher plasma ATP concentration compared to controls, while patients bearing other CF genotypes were similar to controls (n=10, P<0.4). We conclude that CF patients have higher circulating levels of ATP when compared to controls. Increased levels of plasma ATP, which is an important autocrine/paracrine hormone in many cell types, may be associated with chronic manifestations of the disease.

cAMP activates an ATP-permeable pathway in neonatal rat cardiac myocytes

The molecular mechanisms associated with intracellular ATP release by the heart are largely unknown. In this study the luciferin-luciferase assay and patch-clamp techniques were used to characterize the pathways responsible for ATP release in neonatal rat cardiac myocytes (NRCM). Spontaneous ATP release by NRCM was significantly increased after cAMP stimulation under physiological conditions. cAMP stimulation also induced an anion-selective electrodiffusional pathway that elicited linear, diphenylamine-2-carboxylate (DPC)-inhibitable Cl(-) currents in either symmetrical MgCl(2) or NaCl. ATP, adenosine 5'-O-(3-thiotriphosphate), and the ATP derivatives ADP and AMP, permeated this pathway; however, GTP did not. The cAMP-induced ATP currents were inhibited by DPC and glibenclamide and by a monoclonal antibody raised against the R domain of the cystic fibrosis transmembrane conductance regulator (CFTR). The channel-like nature of the cAMP-induced ATP-permeable pathway was also determined by assessing protein kinase A-activated single channel Cl(-) and ATP currents in excised inside-out patches of NRCM. Single channel currents were inhibited by DPC and the anti-CFTR R domain antibody. Thus the data in this report demonstrate the presence of a cAMP-inducible electrodiffusional ATP transport mechanism in NRCM. Based on the pharmacology, patch-clamping data, and luminometry studies, the data are most consistent with the role of a functional CFTR as the anion channel implicated in cAMP-activated ATP transport in NRCM.

cAMP-activated anion conductance is associated with expression of CFTR in neonatal mouse cardiac myocytes

In this study, patch-clamp techniques were applied to cultured neonatal mouse cardiac myocytes (NMCM) to assess the contribution of cAMP stimulation to the anion permeability in this cell model. Addition of either isoproterenol or a cocktail to raise intracellular cAMP increased the whole cell currents of NMCM. The cAMP-dependent conductance was largely anionic, as determined under asymmetrical (low intracellular) Cl(-) conditions and symmetrical Cl(-) in the presence of various counterions, including Na(+), Mg(2+), Cs(+), and N-methyl-D-glucamine. Furthermore, the cAMP-stimulated conductance was also permeable to ATP. The cAMP-activated currents were inhibited by diphenylamine-2-carboxylate, glibenclamide, and an anti-cystic fibrosis transmembrane conductance regulator (CFTR) monoclonal antibody. The anti-CFTR monoclonal antibody failed, however, to inhibit an osmotically activated anion conductance, indicating that CFTR is not linked to osmotically stimulated currents in this cell model. Immunodetection studies of both neonatal mouse heart tissue and cultured NMCM revealed that CFTR is expressed in these preparations. The implication of CFTR in the cAMP-stimulated Cl(-)- and ATP-permeable conductance was further verified with NMCM of CFTR knockout mice [cftr(-/-)] in which cAMP stimulation was without effect on the whole cell currents. In addition, stimulation with protein kinase A and ATP induced Cl(-)-permeable single-channel activity in excised, inside-out patches from control, but not cftr(-/-) NMCM. The data in this report indicate that cAMP stimulation of NMCM activates an anion-permeable conductance with functional properties similar to those expected for CFTR, thus suggesting that CFTR may be responsible for the cAMP-activated conductance. CFTR may thus contribute to the permeation and/or regulation of Cl(-)- and ATP-permeable pathways in the developing heart.

Actin filament organization is required for proper cAMP-dependent activation of CFTR

Previous studies have indicated a role of the actin cytoskeleton in the regulation of the cystic fibrosis transmembrane conductance regulator (CFTR) ion channel. However, the exact molecular nature of this regulation is still largely unknown. In this report human epithelial CFTR was expressed in human melanoma cells genetically devoid of the filamin homologue actin-cross-linking protein ABP-280 [ABP(-)]. cAMP stimulation of ABP(-) cells or cells genetically rescued with ABP-280 cDNA [ABP(+)] was without effect on whole cell Cl(-) currents. In ABP(-) cells expressing CFTR, cAMP was also without effect on Cl(-) conductance. In contrast, cAMP induced a 10-fold increase in the diphenylamine-2-carboxylate (DPC)-sensitive whole cell Cl(-) currents of ABP(+)/CFTR(+) cells. Further, in cells expressing both CFTR and a truncated form of ABP-280 unable to cross-link actin filaments, cAMP was also without effect on CFTR activation. Dialysis of ABP-280 or filamin through the patch pipette, however, resulted in a DPC-inhibitable increase in the whole cell currents of ABP(-)/CFTR(+) cells. At the single-channel level, protein kinase A plus ATP activated single Cl(-) channels only in excised patches from ABP(+)/CFTR(+) cells. Furthermore, filamin alone also induced Cl(-) channel activity in excised patches of ABP(-)/CFTR(+) cells. The present data indicate that an organized actin cytoskeleton is required for cAMP-dependent activation of CFTR.

Role of gelsolin in the actin filament regulation of cardiac L-type calcium channels

The actin cytoskeleton is an important contributor to the modulation of the cell function. However, little is known about the regulatory role of this supermolecular structure in the membrane events that take place in the heart. In this report, the regulation of cardiac myocyte function by actin filament organization was investigated in neonatal mouse cardiac myocytes (NMCM) from both wild-type mice and mice genetically devoid of the actin filament severing protein gelsolin (Gsn-/-). Cardiac L-type calcium channel currents (I(Ca)) were assessed using the whole cell voltage-clamp technique. Addition of the actin filament stabilizer phalloidin to wild-type NMCM increased I(Ca) by 227% over control conditions. The basal I(Ca) of Gsn-/- NMCM was 300% higher than wild-type controls. This increase was completely reversed by intracellular perfusion of the Gsn-/- NMCM with exogenous gelsolin. Further, cytoskeletal disruption of either Gsn-/- or phalloidin-dialyzed wild-type NMCM with cytochalasin D (CD) decreased the enhanced I(Ca) by 84% and 87%, respectively. The data indicate that actin filament stabilization by either a lack of gelsolin or intracellular dialysis with phalloidin increase I(Ca), whereas actin filament disruption with CD or dialysis of Gsn-/- NMCM with gelsolin decrease I(Ca). We conclude that cardiac L-type calcium channel regulation is tightly controlled by actin filament organization. Actin filament rearrangement mediated by gelsolin may contribute to calcium channel inactivation.

Cardiac Gsalpha overexpression enhances L-type calcium channels through an adenylyl cyclase independent pathway

The alpha subunit of the stimulatory heterotrimeric G protein (Gsalpha) is critical for the beta-adrenergic receptor activation of the cAMP messenger system. The role of Gsalpha in regulating cardiac Ca2+ channel activity, however, remains controversial. Cultured neonatal cardiac myocytes from transgenic mice overexpressing cardiac Gsalpha were used to assess the role of Gsalpha on the whole-cell Ca2+ currents (ICa). Cardiac myocytes from transgenic mice had a 490% higher peak ICa compared with those of either wild-type controls or Gsalpha-nonexpressing littermates. The effect of Gsalpha overexpression was mimicked by intracellular dialysis of wild-type cardiac myocytes with GTPgammaS-activated Gsalpha. This effect was not mediated by protein kinase A activation as intracellular perfusion with a protein kinase A inhibitor rendered the same degree of activation in either transgenic or wild-type myocytes also dialyzed with activated Gsalpha. The data indicate that Gsalpha overexpression is associated with a constitutive enhancement of ICa which is independent of the cAMP pathway and activation of endogenous adenylyl cyclase.

Actin cytoskeleton regulates ion channel activity in retinal neurons

The actin cytoskeleton is an important contributor to the integrity of cellular shape and responses in neurons. However, the molecular mechanisms associated with functional interactions between the actin cytoskeleton and neuronal ion channels are largely unknown. Whole-cell and single channel recording techniques were thus applied to identified retinal bipolar neurons of the tiger salamander (Ambystoma tigrinum) to assess the role of acute changes in actin-based cytoskeleton dynamics in the regulation of voltage-gated ion channels. Disruption of endogenous actin filaments after brief treatment (20-30 min) with cytochalasin D (CD) activated voltage-gated K+ currents in bipolar cells, which were largely prevented by intracellular perfusion with the actin filament-stabilizer agent, phalloidin. Either CD treatment under cell-attached conditions or direct addition of actin to excised, inside-out patches of bipolar cells activated and/or increased single K+ channels. Thus, acute changes in actin-based cytoskeleton dynamics regulate voltage-gated ion channel activity in bipolar cells.

Electrodiffusional ATP movement through the cystic fibrosis transmembrane conductance regulator

Expression of the cystic fibrosis transmembrane conductance regulator (CFTR), and of at least one other member of the ATP-binding cassette family of transport proteins, P-glycoprotein, is associated with the electrodiffusional movement of the nucleotide ATP. Evidence directly implicating CFTR expression with ATP channel activity, however, is still missing. Here it is reported that reconstitution into a lipid bilayer of highly purified CFTR of human epithelial origin enables the permeation of both Cl- and ATP. Similar to previously reported data for in vivo ATP current of CFTR-expressing cells, the reconstituted channels displayed competition between Cl- and ATP and had multiple conductance states in the presence of Cl- and ATP. Purified CFTR-mediated ATP currents were activated by protein kinase A and ATP (1 mM) from the "intracellular" side of the molecule and were inhibited by diphenylamine-2-carboxylate, glibenclamide, and anti-CFTR antibodies. The absence of CFTR-mediated electrodiffusional ATP movement may thus be a relevant component of the pleiotropic cystic fibrosis phenotype.

Role of actin filament organization in cell volume and ion channel regulation

The actin cytoskeleton is an intracellular structure, which is involved in the onset and control of cell shape and function. In order for this relevant network to control its own and thus cell volume, specific interactions between the actin cytoskeleton and ion channel regulation controlling intracellular salt and water homeostasis may be invoked. The hypotonic shock-induced, cell volume regulatory decrease (RVD) of most eukaryotic cells is a particularly useful example, as it is initiated and regulated by concerted processes involving both adaptive changes in actin filament organization and bulk fluid extrusion triggered by saline movement and the consequent decrease in cell water. The onset of RVD is linked to the selective activation of osmotically-sensitive ion channels and other relevant ion transport mechanisms involved in the net ionic movement from the cytosol. Such regulatory processes, entailing effector changes in actin filament organization which target the plasma membrane, are largely unknown. In this report, recent studies are summarized implicating dynamic changes in gel properties of the actin cytoskeleton as the effector mechanism in the regulation of ion channel activity, and thus cell volume, in human melanoma cells. Based on the characterization of the hypotonic cell volume regulatory response of human melanoma cells devoid of a functional actin-binding protein (ABP-280, a filamin homolog) and their genetically rescued counterpart transfected with a functional ABP, a hypothesis is raised which is consistent with a regulatory "sensory" mechanism based on the ability of actin networks to respond to changes in the intracellular water-salt homeostasis, which in turn effects signals controlling membrane function, including ion channel activity.

K+ transport and capacitance of the basolateral membrane of the larval frog skin

Skin from larval bullfrogs was mounted in an Ussing-type chamber in which the apical surface was bathed with a Ringer solution containing 115 mM K+ and the basolateral surface was bathed with a Ringer solution containing 115 mM Na+. Ion transport was measured as the short-circuit current (Isc) with a low-noise voltage clamp, and skin resistance (Rm) was measured by applying a direct current voltage pulse. Membrane impedance was calculated by applying a voltage signal consisting of 53 sine waves to the command stage of the voltage clamp. From the ratio of the Fourier-transformed voltage and current signals, it was possible to calculate the resistance and capacitance of the apical and basolateral membranes of the epithelium (Ra and Rb, Ca and Cb, respectively). With SO4(2-) as the anion, Rm decreased rapidly within 5 min following the addition of 150 U/ml nystatin to the apical solution, whereas Isc increased from 0.66 to 52.03 microA/cm2 over a 60-min period. These results indicate that nystatin becomes rapidly incorporated into the apical membrane and that the increase in basolateral K+ permeability requires a more prolonged time course. Intermediate levels of Isc were obtained by adding 50, 100, and 150 U/ml nystatin to the apical solution. This produced a progressive decrease in Ra and Rb while Ca and Cb remained constant. With Cl- as the anion, Isc values increased from 2.03 to 89.57 microA/cm2 following treatment with 150 U/ml nystatin, whereas with gluconate as the anion Isc was only increased from 0.63 to 11.64 microA/cm2. This suggests that the increase in basolateral K+ permeability produced by nystatin treatment, in the presence of more permeable anions, is due to swelling of the epithelial cells of the tissue rather than the gradient for apical K+ entry. Finally, Cb was not different among skins exposed to Cl-, SO4(2-), or gluconate, despite the large differences in Isc, nor did inhibition of Isc by treatment with hyperosmotic dextrose cause significant changes in Cb. These results support the hypothesis that increases in cell volume activate K+ channels that are already present in the basolateral membrane of epithelial cells.

Protein kinase A phosphorylation and G protein regulation of type II pneumocyte Na+ channels in lipid bilayers

Protein kinase A (PKA)- and G protein-mediated regulation of immunopurified adult rabbit alveolar epithelial type II (ATII) cell proteins that exhibit amiloride-sensitive Na+ channel activity was studied in planar lipid bilayers and freshly isolated ATII cells. Addition of the catalytic subunit of PKA + ATP increased single channel open probability from 0.42 +/- 0.05 to 0.82 +/- 0.07 in a voltage-independent manner, without affecting unitary conductance. This increase in open probability of the channels was mainly due to a decrease in the time spent by the channel in its closed state. The apparent inhibition constant for amiloride increased from 8.0 +/- 1.8 microM under control conditions to 15 +/- 3 microM after PKA-induced phosphorylation; that for ethylisopropylamiloride increased from 1.0 +/- 0.4 to 2.0 +/- 0.5 microM. Neither pertussis toxin (PTX) nor guanosine 5'-O-(3-thiotriphosphate) affected ATII Na+ channel activity in bilayers. Moreover, PTX failed to affect amiloride-inhibitable 22Na+ uptake in freshly isolated ATII cells. In vitro, ADP ribosylation induced by PTX revealed the presence of a specifically ribosylated band at 40-45 kDa in the total solubilized ATII cell protein fraction, but not in the immunopurified fraction. Moreover, the immunopurified channel was downregulated in response to guanosine 5'-O-(3-thiotriphosphate)-mediated activation of the exogenous G alpha(i-2), but not G(oA), G alpha(i-1), or G alpha(i-3), protein added to the channel. This effect occurred only in the presence of actin. These results suggest that amiloride-sensitive Na+ channels in adult alveolar epithelia regulated by PKA-mediated phosphorylation also retain the ability to be regulated by G alpha([i-2), but not G alpha([i-1) or G alpha(i-3), protein.

Nucleotide transport through the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the superfamily of ATP-binding cassette (ABC) transporters, also known as traffic ATPases, which are implicated in the movement of various substrates. Recent studies indicate that CFTR and other closely related ABC transporters are also implicated in the movement of cellular ATP. This is the subject of current controversy. Therefore, evidence for the movement of cellular nucleotides by expression of CFTR and related molecules, as well as the potential significance of ATP-permeable channels in cell physiology, are reviewed in this study. The hypothesis is thus forwarded for the improper delivery of cellular ATP to the extracellular milieu by a dysfunctional CFTR, to be a relevant factor in the onset of cystic fibrosis.

Role of actin in regulation of epithelial sodium channels by CFTR

Cystic fibrosis (CF) airway epithelia exhibit enhanced Na+ reabsorption in parallel with diminished Cl- secretion. We tested the hypothesis that actin plays a role in the regulation of a cloned epithelial Na+ channel (ENaC) by the cystic fibrosis transmembrane conductance regulator (CFTR). We found that immunopurified bovine tracheal CFTR coreconstituted into a planar lipid bilayer with alpha,beta,gamma-rat ENaC (rENaC) decreased single-channel open probability (Po) of rENaC in the presence of actin by over 60%, a significantly greater effect than was observed in the absence of actin (approximately 20%). In the presence of actin, protein kinase A plus ATP activated both CFTR and rENaC, but CFTR was activated in a sustained manner, whereas the activation of rENaC was transitory. ATP alone could also activate ENaC transiently in the presence ofactin but had no effect on CFTR. Stabilizing short actin filaments at a fixed length with gelsolin (at a ratio to actin of 2:1) produced a sustained activation of alpha,beta,gamma-rENaC in both the presence or absence of CFTR. Gelsolin alone (i.e., in the absence of actin) had no effect on the conductance or Po of either CFTR or rENaC. We have also found that short actin filaments produced their modulatory action on alpha-rENaC independent of the presence of the beta- or gamma-rENaC subunits. In contrast, CFTR did not affect any properties of the channel formed by alpha-rENaC alone, i.e., in the absence of beta- or gamma-rENaC. These results indicate that CFTR can directly downregulate single Na+ channel activity, which may account for the observed differences between Na+ transport in normal and CF-affected airway epithelia. Moreover, the presence of actin confers an enhanced modulatory ability of CFTR on Na+ channels.

cAMP activates an ATP-conductive pathway in cultured shark rectal gland cells

The molecular mechanisms associated with ATP transport and release into the extracellular milieu are largely unknown. To assess the presence of endogenous ATP-conductive pathway(s) in shark rectal gland (SRG) cells, patch-clamp techniques were applied to primary cultures of SRG cells. Whole cell currents were obtained with either intracellular tris(hydroxymethyl)aminomethane (Tris) or Mg2+ salts of ATP (200 mM nominal ATP) and 280 mM NaCl bathing solution. Basal currents showed a sizable ATP permeability for outward movement of MgATP. Adenosine 3',5'-cyclic monophosphate (cAMP) stimulation significantly increased the whole cell conductance (with either intracellular Tris-ATP or MgATP). Symmetrical whole cell ATP currents were also observed after cAMP activation, thus consistent with ATP as the main charge carrier. The cAMP-inducible ATP currents were insensitive to the Cl- channel blockers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, diphenylamine-2-carboxylate, and anthracene-9-carboxylic acid but were readily blocked by nifedipine (400 microM) and glibenclamide (400 microM). The nature of the electrodiffusional ATP movement was further assessed by single-channel analysis of either MgATP or Tris-ATP currents in excised inside-out patches, both spontaneous and after activation with protein kinase A. Single-channel ATP currents were inhibited by either nifedipine or glibenclamide. Thus SRG cells express endogenous ATP-permeable pathways both before and after cAMP stimulation. Electrodiffusional ATP movement by SRG cells may play a significant role in the transport and delivery of cellular ATP to the extracellular milieu, which may help coordinate the dynamics of the epithelial secretory response in this cell model.

Expression of Drosophila melanogaster P-glycoproteins is associated with ATP channel activity

Two distinct Drosophila melanogaster P-glycoprotein (Pgp) gene homologues of different chromosomal origin, MDR49 and MDR65, have been previously identified (38). Most Pgps are implicated in the development of the multidrug-resistance phenotype. Despite intense efforts to identify the molecular mechanism(s) associated with Pgp function, the endogenous substrate(s) of these transport molecules is largely unknown. Recent studies from our laboratory indicate that a murine Pgp homologue (E. H. Abraham, A. G. Prat, L. Gerweck, T. Seneveratne, R. J. Arceci, R. Kramer, G. Guidotti, and H. F. Cantiello. Proc. Natl. Acad. Sci. USA 90: 312-316, 1993) and a related protein, the cystic fibrosis transmembrane conductance regulator (CFTR; I. L. Reisin, A. Prat, E. H. Abraham, J. F. Amara, R. J. Gregory, D. A. Ausiello, and H. F. Cantiello. J. Biol. Chem. 269: 20584-20591, 1994), are novel ATP-permeable ion channels. The common feature of these two proteins is the conserved ATP-binding cassettes (ABC); thus molecules structurally linked to the ABC transporter family may be also functionally associated with ATP channel activity. In this study, MDR65 and MDR49 Pgps were functionally expressed in Sf9 cells, and patch-clamp techniques were applied to assess the role of these proteins in the electrodiffusional movement of ATP. In the presence of intracellular ATP and external NaCl, expression of MDR65 was associated with a linear electrodiffusional pathway that was permeable to both ATP and Cl-. Under symmetrical ATP conditions, only voltage depolarization activated a MDR65-mediated ATP-conductive pathway. Expression of MDR49 was also associated with a voltage-activated ATP conductance in symmetrical ATP, but no apparent permeability to either Cl- or ATP was observed under asymmetrical conditions. The different functional properties of MDR65 and MDR49 may be indicative of distinct physiological roles in this organism. The study indicates, however, that the two Drosophila Pgp homologues share strong functional similarities with their mammalian relatives Pgp and CFTR.

Renal epithelial protein (Apx) is an actin cytoskeleton-regulated Na+ channel

Apx, the amphibian protein associated with renal amiloride-sensitive Na+ channel activity and with properties consistent with the pore-forming 150-kDa subunit of an epithelial Na+ channel complex initially purified by Benos et al. (Benos, D. J., Saccomani, G., and Sariban-Sohraby, S.(1987) J. Biol. Chem. 262, 10613-10618), has previously failed to generate amiloride-sensitive Na+ currents (Staub, O., Verrey, F., Kleyman, T. R., Benos, D. J., Rossier, B. C., and Kraehenbuhl, J.-P.(1992) J. Cell Biol. 119, 1497-1506). Renal epithelial Na+ channel activity is tonically inhibited by endogenous actin filaments (Cantiello, H. F., Stow, J., Prat, A. G., and Ausiello, D. A.(1991) Am. J. Physiol. 261, C882-C888). Thus, Apx was expressed and its function examined in human melanoma cells with a defective actin-based cytoskeleton. Apx-transfection was associated with a 60-900% increase in amiloride-sensitive (Ki = 3 microM) Na+ currents. Single channel Na+ currents had a similar functional fingerprint to the vasopressin-sensitive, and actin-regulated epithelial Na+ channel of A6 cells, including a 6-7 pS single channel conductance and a perm-selectivity of Na+:K+ of 4:1. Na+ channel activity was either spontaneous, or induced by addition of actin or protein kinase A plus ATP to the bathing solution of excised inside-out patches. Therefore, Apx may be responsible for the ionic conductance involved in the vasopressin-activated Na+ reabsorption in the amphibian kidney.

Regulation of epithelial sodium channels by short actin filaments

Cytoskeletal elements play an important role in the regulation of ion transport in epithelia. We have studied the effects of actin filaments of different length on the alpha, beta, gamma-rENaC (rat epithelial Na+ channel) in planar lipid bilayers. We found the following. 1) Short actin filaments caused a 2-fold decrease in unitary conductance and a 2-fold increase in open probability (Po) of alpha,beta,gamma-rENaC. 2) alpha,beta,gamma-rENaC could be transiently activated by protein kinase A (PKA) plus ATP in the presence, but not in the absence, of actin. 3) ATP in the presence of actin was also able to induce a transitory activation of alpha, beta,gamma-rENaC, although with a shortened time course and with a lower magnitude of change in Po. 4) DNase I, an agent known to prohibit elongation of actin filaments, prevented activation of alpha,beta,gamma-rENaC by ATP or PKA plus ATP. 5) Cytochalasin D, added after rundown of alpha,beta,gamma-rENaC activity following ATP or PKA plus ATP treatment, produced a second transient activation of alpha,beta,gamma-rENaC. 6) Gelsolin, a protein that stabilizes polymerization of actin filaments at certain lengths, evoked a sustained activation of alpha,beta,gamma-rENaC at actin/gelsolin ratios of <32:1, with a maximal effect at an actin/gelsolin ratio of 2:1. These results suggest that short actin filaments activate alpha, beta,gamma-rENaC. PKA-mediated phosphorylation augments activation of this channel by decreasing the rate of elongation of actin filaments. These results are consistent with the hypothesis that cloned alpha,beta,gamma-rENaCs form a core conduction unit of epithelial Na+ channels and that interaction of these channels with other associated proteins, such as short actin filaments, confers regulation to channel activity.

cAMP-independent regulation of CFTR by the actin cytoskeleton

Pages C1552-C1561: A. G. Prat, Y.-F. Xiao, D. A. Ausiello, and H. F. Cantiello. “AMP-independent regulation of CFTR by the actin cytoskeleton.” Page C1558, Fig. 8A: units on the y-axis of the bar graph should have been 10 times larger. The corrected figure appears below.

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Nuclear ion channel activity is regulated by actin filaments

Actin filaments are novel second messengers involved in ion channel regulation. Because cytoskeletal components interact with the nuclear envelope, the actin cytoskeleton may also control nuclear membrane function. In this report, the patch-clamp technique was applied to isolated nuclei from amphibian A6 epithelial cells to assess the role of actin filaments on nuclear ion channel activity under nucleus-attached or -excised conditions. The most prevalent spontaneous nuclear ion channel species, 76% (n = 46), was cation selective and had a maximal single-channel conductance of approximately 420 pS. Nuclear ion channels also displayed multiple subconductance states, including channel activity of 26 pS that was frequently observed. Nuclear ion channel activity on otherwise quiescent patches was induced by either addition of the actin cytoskeleton disrupter cytochalasin D (CD; 5 micrograms/ml, 60%, 3 of 5 patches) or actin (10-1,000 micrograms/ml) to the bathing solution of nucleus-attached patches (59%, 13 of 22 patches). Actin also induced ion channel activity in quiescent excised inside-out patches from the nuclear envelope (80%, 4 of 5 patches). In contrast, addition of bovine serum albumin (10-1,000 micrograms/ml) to the bathing solution of nucleus-attached patches was without effect on nuclear ion channel activity (5 of 5 patches). The monoclonal antibody MAb414, specific for nuclear pore complex proteins, completely prevented either spontaneous or cytosolic actin-induced nuclear ion channels under nucleus-attached conditions (4 of 4 patches) but not intranuclear actin-induced nuclear ion channels under excised inside-out conditions (3 of 3 patches). In nucleus-attached patches, channel activity was readily activated by addition of the G-actin-binding protein deoxyribonuclease I to nucleus-attached patches (56%, 5 of 9 patches) or further addition of the actin-cross-linker filamin in the presence of actin (57%, 4 of 7 patches). The data indicate that dynamic changes in actin filament organization may represent a novel mechanism to control nuclear function.

Role of the actin cytoskeleton in the regulation of the cystic fibrosis transmembrane conductance regulator

The anion-selective channel CFTR (cystic fibrosis transmembrane conductance regulator), whose dysfunction is responsible for the onset of cystic fibrosis, is regulated by cAMP through the activation of protein kinase A (PKA). The nature of this activation process is unknown. In the present study, patch-clamp techniques were applied to both mouse mammary adenocarcinoma cells expressing human epithelial CFTR (CFTR cells) and cultured neonatal rat ventricular myocytes (NRVM), to determine whether CFTR is modulated by the actin cytoskeleton, and whether the actin cytoskeleton may be implicated in the cAMP-stimulated activation of the channel protein. Acute changes in the actin cytoskeleton by addition of cytochalasin D (CD) activated whole-cell currents in CFTR cells and NRVM. Addition of actin to excised, inside-out patches also activated CFTR. A functional characterization of CFTR in either cell type included cAMP-induced, linear whole-cell and single-channel currents in symmetrical Cl-, permeability to ATP, and inhibition by either diphenylamine-carboxylate (DPC) or a monoclonal antibody raised against CFTR. Incubation of CFTR cells and NRVM with CD for over 6 h prevented CFTR activation either by the cAMP pathway under whole-cell conditions or by PKA under excised inside-out conditions. Thus a complete derangement of the actin cytoskeleton prevents the cAMP-dependent activation of CFTR. CFTR activation, however, was restored by subsequent addition of actin. In summary, changes in actin filament organization modulate CFTR channel activity by a mechanism entailing a direct interaction between actin filaments and CFTR.

cAMP-independent regulation of CFTR by the actin cytoskeleton

Pages C1552<C1561: A. G. Prat, Y.-F. Xiao, D. A. Ausiello, and H. F. Cantiello. “cAMP-independent regulation of CFTR by the actin cytoskeleton.” Page C1558, Fig. 8A: units on the y-axis of the bar graph should have been 10 times larger. The corrected figure appears below.

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Cellular ATP release by the cystic fibrosis transmembrane conductance regulator

Recent studies from our laboratory indicate that members of the ATP-binding cassette (ABC) family of transporters, including P-glycoprotein and cystic fibrosis transmembrane conductance regulator (CFTR), are ATP-permeable channels. The physiological relevance of this novel transport mechanism is largely unknown. In the present study, intra- and extracellular ATP content, cellular ATP release, and O2 consumption before and after adenosine 3',5'-cyclic monophosphate (cAMP) stimulation were determined to assess the role of CFTR in the transport of ATP under physiological conditions. The functional expression of CFTR by the stable transfection of mouse mammary carcinoma cells, C1271, with human epithelial CFTR cDNA resulted in a stimulated metabolism, since both basal and cAMP-inducible O2 consumption were increased compared with mock-transfected cells. The stimulated (but not basal) O2 consumption was inhibited by diphenyl-2-carboxylic acid (DPC), a known inhibitor of CFTR. CFTR expression was also associated with the cAMP-activated and DPC-inhibitable release of intracellular ATP. The recovery of intracellular ATP from complete depletion after metabolic poisoning was also assessed under basal and cAMP-stimulated conditions. The various maneuvers indicate that CFTR may be an important contributor to the release of cellular ATP, which may help modify signal transduction pathways associated with secretory Cl- movement or other related processes. Changes in the CFTR-mediated delivery of nucleotides to the extracellular compartment may play an important role in the onset and reversal of the cystic fibrosis phenotype.

Actin filaments stimulate the Na(+)-K(+)-ATPase

In this report, the functional role of actin on Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) activity was explored. The Na(+)- and K(+)-dependent, ouabain-sensitive ATP hydrolysis mediated by rat kidney Na(+)-K(+)-ATPase increased by 74% in the presence of previously unpolymerized actin (24 microM), whereas addition of polymerized actin was without effect. Addition of actin was associated instead with an increase in the affinity of the Na(+)-K(+)-ATPase for Na+ but not other enzymatic substates. A maximal stimulatory effect (296%) was observed either at an Na(+)-K(+)-ATPase:actin ratio of 1:50,000 or at lower ratios (1:625) by shifting from the E2 (K+ selective) to the E1 (Na+ selective) conformation of the enzyme. Immunoblotting of actin to the purified Na(+)-K(+)-ATPase suggested that this interaction may be linked to binding of actin to the enzyme, which was further supported by sequence analysis indicating putative actin-binding domains in the alpha-subunit of the enzyme. The interaction between actin and the Na(+)-K(+)-ATPase may imply a novel functional role of the cytoskeleton in the control of ion transport.

Role of the actin cytoskeleton on epithelial Na+ channel regulation

The regulatory role of actin filament organization on epithelial Na+ channel activity is reviewed in this report. The actin cytoskeleton, consisting of actin filaments and associated actin-binding proteins, is essential to various cellular events including the maintenance of cell shape, the onset of cell motility, and the distribution and stability of integral membrane proteins. Functional interactions between the actin cytoskeleton and specific membrane transport proteins are, however, not as well understood. Recent studies from our laboratory have determined that dynamic changes in the actin cytoskeletal organization may represent a novel signaling mechanism in the regulation of ion transport in epithelia. This report summarizes work conducted in our laboratory leading to an understanding of the molecular steps associated with the regulatory role of the actin-based cytoskeleton on epithelial Na+ channel function. The basis of this interaction lies on the regulation by actin-binding proteins and adjacent structures, of actin filament organization which in turn, modulates ion channel activity. The scope of this interaction may extend to such relevant cellular events as the vasopressin response in the kidney.

cAMP-independent regulation of CFTR by the actin cytoskeleton

Protein kinase A (PKA)-activation of epithelial Na+ channels requires actin filaments. Mouse mammary adenocarcinoma cells expressing the human cystic fibrosis transmembrane conductance regulator (CFTR) or mock transfectants were used to determine whether CFTR is also modulated by the actin cytoskeleton. The actin filament disrupter cytochalasin D (CD; approximately 5 micrograms/ml) readily activated whole cell currents in CFTR but not in mock-transfected (MOCK) cells. Addition of actin to the cytosolic side of quiescent excised inside-out patches of CFTR but not MOCK cells also activated CFTR. The actin-activated Cl- channels (symmetrical Cl-) had a linear conductance of 9.3 pS and were inhibited by diphenylamine-2-carboxylate and monoclonal antibodies raised against CFTR. Channel activity was also blocked by addition of the actin-binding proteins deoxyribonuclease I and filamin. Incubation of CFTR cells with CD (approximately 15 micrograms/ml) for > 6 h prevented CFTR activation by the addition of either 8-bromoadenosine 3',5'-cyclic monophosphate plus forskolin under whole cell conditions or PKA under excised inside-out conditions. However, CFTR activation was restored by subsequent addition of actin. The data indicate that CFTR is regulated by actin filaments whose effect may, in turn, be associated with the PKA-dependent pathway.

The cystic fibrosis transmembrane conductance regulator is a dual ATP and chloride channel

The cystic fibrosis transmembrane conductance regulator (CFTR) belongs to a superfamily of proteins implicated in the transport of ions, proteins, and hydrophobic substances. Recent studies have demonstrated that CFTR is a protein kinase A-sensitive anion channel regulated by ATP. In the present study, patch-clamp techniques were used to assess the role of CFTR in the transport of Cl- and ATP. The stable transfection of mouse mammary carcinoma cells, C127i, with the cDNA for human CFTR resulted in the appearance of a diphenylamine-2-carboxylate-inhibitable Cl- channel, which was activated by cAMP under whole-cell and cell-attached conditions and by protein kinase A plus ATP under excised, inside-out conditions. CFTR expression was also associated with the electrodiffusional movement of ATP as indicated by the cAMP activation of ATP currents measured under whole-cell conditions. In excised, inside-out patches, it was demonstrated that ATP currents were mediated by ATP-conductive channels, which were also activated by protein kinase A and blocked by the Cl- channel blocker diphenylamine-2-carboxylate under excised, inside-out conditions. Single-channel currents observed in the presence of asymmetrical Cl-/ATP concentrations indicated that the same conductive pathway was responsible for both ATP and Cl- movement. Thus, CFTR is a multifunctional protein with more than one anion transport capability and may modify signal transduction pathways for Cl- or other secretory processes by the selective delivery of nucleotides to the extracellular domain.

External ATP and its analogs activate the cystic fibrosis transmembrane conductance regulator by a cyclic AMP-independent mechanism

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl- channel activated by protein kinase A and regulated by ATP in a complex manner. We have applied patch-clamp techniques to C127i mouse mammary carcinoma cells transfected with human CFTR to assess the role of external ATP in the modulation of CFTR function. Extracellular ATP was sufficient to activate non-rectifying, Cl(-)-selective whole-cell currents in CFTR-transfected, but not mock-transfected cells. The ATP-mediated activation of CFTR was independent of protein kinase A since channel activation by ATP was preserved in cells that were (a) depleted of intracellular ATP, (b) incubated with the cAMP antagonist Rp-cAMPS, or (c) exposed to the protein kinase A inhibitor, 5-24 amide. In each of these conditions, 8-Br-cAMP was no longer capable of activating CFTR. The possibility that the extracellular ATP activation of Cl- currents in CFTR-expressing C127i cells was mediated by a P2-type purinergic receptor was supported by studies in which the effect of external ATP on the Cl- currents was mimicked by the ATP analogs, ATP gamma S and beta,gamma-methylene ATP, but not the uridine nucleotide, UTP. Single-channel analysis of ATP-activated Cl -currents under both cell-attached and excised, inside-out patch-clamp configurations indicated that this channel is only present in CFTR-transfected cells and indistinguishable from CFTR. External ATP also activated ATP currents in CFTR-transfected cells, a novel function of CFTR. These findings are consistent with the presence of a purinergic receptor signal transduction mechanism in C127i cells whose activation by external ATP is linked to the activation of CFTR in a cAMP-independent manner. The data provide additional support for the use of ATP and its analogs as alternative therapies in cystic fibrosis.

A novel method to study the electrodynamic behavior of actin filaments. Evidence for cable-like properties of actin

Actin, one of the most abundant intracellular proteins, forms long linear polyelectrolytic polymers in solution. A novel technique to handle single actin filaments in solution was developed that allows the study of ionic currents elicited along the surface of electrically stimulated actin filaments. Electrical currents were observed about the polymer's surface under both high (100 mM KCl) and low (1 mM KCl) ionic strength conditions. The data are consistent with a dynamic behavior of the counterionic cloud surrounding the actin filaments that support ionic movements along their longitudinal axis upon electrical stimulation. Counterionic waves were highly nonlinear in nature and remained long after the electrical stimulation of the actin filaments ceased. In this report therefore, we demonstrate that actin filaments can function as biological "electrical wires" and can thus be conceptualized as nonlinear inhomogeneous transmission lines. This ability of actin filaments to conduct electrical signals may have important implications in the coupling of intracellular signals.

Vasopressin and protein kinase A activate G protein-sensitive epithelial Na+ channels

To determine the molecular steps involved in the vasopressin-induced renal Na+ reabsorption, the patch-clamp technique was utilized to study the role of this hormone in the regulation of apical Na+ channels in renal epithelial A6 cells. Addition of arginine vasopressin (AVP) induced and/or enhanced Na+ channel activity within 5 min of addition under cell-attached conditions. The AVP-induced channel activity was a reflection of both an increase in the average apparent channel number (0.2-1.7) and the percent open time (2-56%). Addition of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, the adenosine 3',5'-cyclic monophosphate (cAMP) analogues, 8-(4-chlorophenylthio)-cAMP and 8-bromo-cAMP, or forskolin elicited a comparable effect to that of AVP. The induced channels had similar properties to Na+ channels previously reported, including a channel conductance of 9 pS, Na(+)-to-K+ selectivity of 3-5:1, and high amiloride sensitivity. The cAMP-dependent protein kinase A (PKA) in the presence of ATP induced and/or enhanced Na+ channel activity in excised inside-out patches with a change in average apparent channel number and percent open probability similar to those observed with either AVP or cAMP analogues in intact cells. Addition of activated pertussis toxin (100 ng/ml) completely blocked the AVP- or PKA-induced Na+ channel activity in excised inside-out patches, whereas incubation of intact cells with the toxin completely prevented the effect of both activators. The data indicate that AVP mediates its effect through a cAMP-dependent pathway involving PKA activation whose target is the G protein pathway that regulates apical epithelial Na+ channel activity.

Activation of epithelial Na+ channels by protein kinase A requires actin filaments

We have recently demonstrated a novel role for "short" actin filaments, a distinct species of polymerized actin different from either monomeric (G-actin) or long actin filaments (F-actin), in the activation of epithelial Na+ channels. In the present study, the role of actin in the activation of apical Na+ channels by the adenosine 3',5'-cyclic monophosphate-dependent protein kinase A (PKA) was investigated by patch-clamp techniques in A6 epithelial cells. In excised inside-out patches, addition of deoxyribonuclease I, which prevents actin polymerization, inhibited Na+ channel activation mediated by PKA. Disruption of endogenous actin filament organization with cytochalasin D for at least 1 h prevented the PKA-mediated activation of Na+ channels but not activation following the addition of actin to the cytosolic side of the patch. To assess the role of PKA on actin filament organization, actin was used as a substrate for the specific phosphorylation by the PKA. Actin was phosphorylated by PKA with an equilibrium stoichiometry of 2:1 mol PO4-actin monomer. Actin was phosphorylated in its monomeric form, but only poorly once polymerized. Furthermore, phosphorylated actin reduced the rate of actin polymerization. Thus actin allowed to polymerize for at least 1 h in the presence of PKA and ATP to obtain phosphorylated actin filaments induced Na+ channel activity in excised inside-out patches, in contrast to actin polymerized either in the absence of PKA or in the presence of PKA plus a PKA inhibitor (nonphosphorylated actin filaments). This was also confirmed by using purified phosphorylated G-actin incubated in a polymerizing buffer for at least 1 h at 37 degrees C. These data suggest that the form of actin required for Na+ channel activation (i.e., "short" actin filaments) may be favored by the phosphorylation of G-actin and may thus mediate or facilitate the activation of Na+ channels by PKA.

Actin-binding protein contributes to cell volume regulatory ion channel activation in melanoma cells

The cell volume regulatory response to a hypotonic stimulus is frequently initiated by activation of K+ and Cl- channels. We have characterized the hypotonic cell volume regulatory response of human melanoma cells devoid of actin-binding protein (ABP) and their genetically rescued counterpart transfected with the cDNA for ABP. ABP-deficient cells were unable to volume-regulate or activate K+ channels when exposed to a hypotonic stimulus. Genetic rescue with ABP resulted in recovery of both the cell volume regulatory response and the osmotically linked K+ channel activation. These data are consistent with a functional interaction between the actin cytoskeleton and osmotically sensitive ion transport.

The multidrug resistance (mdr1) gene product functions as an ATP channel

The multidrug resistance (mdr1) gene product, P-glycoprotein, is responsible for the ATP-dependent extrusion of a variety of compounds, including chemotherapeutic drugs, from cells. The data presented here show that cells with increased levels of the P-glycoprotein release ATP to the medium in proportion to the concentration of the protein in their plasma membrane. Furthermore, measurements of whole-cell and single-channel currents with patch-clamp electrodes indicate that the P-glycoprotein serves as an ATP-conducting channel in the plasma membrane. These findings suggest an unusual role for the P-glycoprotein.

Purified epithelial Na+ channel complex contains the pertussis toxin-sensitive G alpha i-3 protein

We have recently demonstrated that the amiloride-sensitive Na+ channel in the apical membrane of the renal epithelial cell line, A6, is modulated by the alpha i-3 subunit of the Gi-3 protein. We also showed that a 700-kDa protein complex can be purified from the membranes of A6 epithelia which (a) can reconstitute the amiloride-sensitive Na+ influx in liposomes and planar bilayer membranes and (b) consists of six major protein bands observed on reducing sodium dodecyl sulfate-polyacrylamide gels with molecular masses ranging from 35 to 320 kDa. The present study was undertaken to determine if the alpha i-3 subunit was a member of this Na+ channel complex. G alpha i structure and function were identified by Western blotting with specific G alpha i subunit antibodies and Na+ channel antibodies, through ADP-ribosylation with pertussis toxin, and by immunocytochemical localization of the Na+ channel and G alpha i proteins. We demonstrate that two protein substrates are ADP-ribosylated in the 700-kDa complex in the presence of pertussis toxin and are specifically immunoprecipitated with an anti-Na+ channel polyclonal antibody. One of these substrates, a 41-kDa protein, was identified as the alpha i-3 subunit of the Gi-3 protein on Western blots with specific antibodies. Na+ channel antibodies do not recognize G alpha i-3 on Western blots of Golgi membranes which contain alpha i-3 but not Na+ channel proteins, nor do they immunoprecipitate alpha i-3 from solubilized Golgi membranes; however, alpha i-3 is coprecipitated as part of the Na+ channel complex from A6 cell membranes by polyclonal Na+ channel antibodies. Both alpha i-3 and the Na+ channel have been localized in A6 cells by confocal imaging and immunofluorescence with specific antibodies and are found to be in distinct but adjacent domains of the apical cell surface. In functional studies, alpha i-3, but not alpha i-2, stimulates Na+ channel activity. These data are therefore consistent with the localization of Na+ channel activity and modulatory alpha i-3 protein at the apical plasma membrane, which together represent a specific signal transduction pathway for ion channel regulation.

Actin filaments regulate epithelial Na+ channel activity

The functional role of the cytoskeleton in the control of ion channel activity is unknown. In the present study, immunocolocalization of Na+ channels with specific antibodies and fluorescein isothiocyanate-phalloidin to stain the cortical cytoskeleton indicates that actin is always present in close proximity to apical Na+ channels in A6 cells. The patch-clamp technique was used to assess the effect of cortical actin networks on apical Na+ channels in these A6 epithelial cells. The actin filament disrupter, cytochalasin D (5 micrograms/ml), induced Na+ channel activity in cell-attached patches within 5 min of addition. Cytochalasin D also induced and/or increased Na+ channel activity in 90% of excised patches tested within 2 min. Addition of short actin filaments (greater than 5 microM) to excised patches also induced channel activity. This effect was enhanced by addition of ATP and/or cytochalasin D. The effect of actin on Na+ channel activity was reversed by addition of the G actin-binding protein DNase I or completely prevented by treatment of the excised patches with this enzyme. Addition of the actin-binding protein, filamin, reversibly inhibited both spontaneous and actin-induced Na+ channels. Thus actin filament networks, achieved by either depolymerizing endogenous actin filaments by treatment with cytochalasin D, the addition of exogenous short actin filaments plus ATP, or actin plus cytochalasin D, regulate apical Na+ channel activity. This conclusion was supported by the observation that the addition of short actin filaments in the form of actin-gelsolin complexes in molar ratios less than 8:1 was also effective in activating Na+ channels. We have thus demonstrated a functional role for the cortical actin network in the regulation of epithelial Na+ channels that may complement a structural role for membrane protein targetting and assembly.

Osmotically induced electrical signals from actin filaments

Actin filaments, F-actin, a major component of the cortical cytoskeleton, play an important role in a variety of cell functions. In this report we have assessed the role of osmotic stress on the electrochemical properties of F-actin. The spontaneous Donnan potential of a polymerized actin solution (5 mg/ml) was -3.93 +/- 1.84 mV, which was linearly reduced by osmotic stress on the order of 1-20 mOsm (0.28 +/- 0.06 mV/mM). Calculated surface charge density was reduced and eventually reversed by increasing the osmotic stress as expected for a phase transition behavior. The electro-osmotic behavior of F-actin disappeared at pH 5.5 and was dependent on its filamentous nature. Furthermore, osmotically stressed F-actin displayed a nonlinear electric response upon application of electric fields on the order of 500-2,000 V/cm. These data indicate that F-actin in solution may display nonideal electro-osmotic properties consistent with ionic "cable" behavior which may be of biological significance in the processing and conduction of electrical signals within the cellular compartment.

G alpha i-3 regulates epithelial Na+ channels by activation of phospholipase A2 and lipoxygenase pathways

Polarized renal epithelial cells have pertussis toxin-sensitive Gi proteins at their apical membrane capable of modulating Na+ channel activity (Cantiello, H.F., Patenaude, C.R., and Ausiello, D.A. (1989) J. Biol. Chem. 264, 20867-20870). In this study, the patch clamp technique was used to assess if this Gi-mediated regulation of Na+ channels is a component of a phospholipid signal transduction pathway. In excised inside-out patches of apical membranes of A6 cells, guanosine 5'-(3-O-thio)triphosphate (GTP gamma S)-stimulated Na+ channel activity (percent open time and channel number) was inhibited by the phospholipase inhibitor mepacrine (50 microM), which had no effect on single channel conductance. In contrast, Na+ channel activity increased in a Ca2(+)-dependent manner following the addition of 100 nM mellitin to untreated or pertussis toxin-treated patches. Addition of 10 microM arachidonic acid in the presence of mepacrine increased Na+ channel activity. Both percent open time and Na+ channel number induced by GTP gamma S, the exogenous alpha i-3 subunit, or arachidonic acid were inhibited by the addition of the 5-lipoxygenase inhibitor nordihydroguaiaretic acid. Na+ channel activity was restored with the addition of leukotriene D4 (100 nM) or the parental leukotriene substrate 5-hydroperoxyeicosatetraenoic acid (10 microM). Thus, Gi activation of apical membrane epithelial Na+ channels is mediated through the regulation of phospholipase and lipoxygenase activities. This apically located signal transduction pathway may be sensitive to, or independent of, classical second messengers generated at the basolateral membrane and known to be responsible for modulation of Na+ channel activity in epithelia.

Phorbol ester-stimulated human neutrophil membrane depolarization is dependent on Ca2(+)-regulated Cl- efflux

The ionic basis of phorbol 12-myristate 13-acetate (PMA)-stimulated membrane depolarization in the human neutrophil has not previously been established. Alterations in cation permeability are probably not directly responsible for the depolarization response, since the rate or Rb+ influx or efflux is unchanged upon PMA stimulation, and although Na+ fluxes are increased, depolarization is not changed by either the addition of ouabain or reduction of extracellular Na+ from 140 to 0 meq. Furthermore, the enhanced Na+ influx in stimulated cells is blocked by amiloride at 10(-3) M, but not by 10(-5) M, suggesting Na+ influx proceeds through the electroneutral Na(+)-H+ antiporter and is therefore not responsible for depolarization. Upon stimulation, Cl- content of PMA-stimulated neutrophils decreases without change in Na+ or K+ content, as determined by electron probe analysis. In addition, acute reduction in extracellular Cl- enhances the rate and extent of depolarization induced by PMA. This change in intracellular Cl- and effect of reduction in extracellular Cl- concentration on depolarization can best be accounted for by an enhanced efflux via an electrogenic mechanism. Thus enhanced conductive Cl- efflux can account for the observed depolarization. That Ca2+ regulates depolarization is evidenced by the dependence of depolarization on external Ca2+ (Cao2+). Depolarization is absent in Ca2(+)-depleted cells [internal Ca2+ (Cai2+) less than 15 nM] and is restored with titration of extracellular Ca2+, exhibiting a 50% effective dose (ED50) of 100 mM. Thus PMA-initiated depolarization is regulated by Ca2+, either from intra- or extracellular sources, but the Ca2(+)-dependent activity responsible for control of Cl- efflux is as yet uncharacterized.

G protein subunit, alpha i-3, activates a pertussis toxin-sensitive Na+ channel from the epithelial cell line, A6

In nonpolar excitable cells, guanine nucleotide regulatory (G) proteins have been shown to modulate ion channel activity in response to hormone receptor activation. In polarized epithelia, hormone receptor-G protein coupling involved in the generation of cAMP occurs on the basolateral membrane, while the physiological response to this messenger is a stimulation of ion channel activity at the apical membrane. In the present study we have utilized the patch-clamp technique to assess if the polarized renal epithelia, A6, have topologically distinct G proteins at their apical membrane capable of modulating Na+ channel activity. In excised inside-out patches of apical membranes, spontaneous Na+ channel activity (conductance 8-9 picosiemens) was inhibited by the addition of 0.1 mM guanosine 5'-O-(2-thio)diphosphate to the cytosolic membrane surface without an effect on single channel conductance. In contrast, the percent open time of spontaneous Na+ channels increased from 6 to 50% following the addition of 0.1 mM GTP. The addition of preactivated pertussis toxin (100 ng/ml) to the cytosolic bathing solution of the excised patch inhibited spontaneous Na+ channel activity within a minute by 85% from approximately 47 to 7% open time and reduced the percent open time for Na+ channel activity to zero after approximately 3 min. The addition of 0.1 mM guanosine 5'-(3-O-thio)triphosphate or the addition of 20 pM purified human alpha i-3 subunit to pertussis toxin-treated membrane patches restored Na+ channel activity from zero to 35% open time. As little as 0.2 pM alpha i-3 subunit was capable of restoring Na+ channel activity. These data provide evidence for a role of pertussis toxin-sensitive G proteins in the apical plasma membrane of renal epithelia distal to signal transduction pathways in the basolateral membrane of these cells. This raises the possibility of a topologically distinct signal transducing pathway co-localized with the Na+ channel.