Inactivation of the alamethicin-induced conductance caused by quaternary ammonium ions and local anesthetics

Long alkyl chain quaternary ammonium ions (QA), the local anesthetics (LA) tetracaine and lidocaine, imipramine, and pancuronium cause inactivation of the alamethicin-induced conductance in lipid bilayer membranes. The alamethicin-induced conductance undergoes inactivation only when these amphipathic compounds are added to the side containing alamethicin. The concentration of QA required to cause a given amount of inactivation depends on the length of the hydrocarbon chain and follows the sequence C9 greater than C10 greater than C12 greater than C16. LA and imipramine, in contrast to QA or pancuronium, are able to promote appreciable inactivation only if the pH of the alamethicin-free side is equal to or lower than the pK of these compounds. The membrane permeability to QA, LA, or imipramine is directly proportional to the alamethicin-induced conductance and is larger than the one for potassium. The observed steady state and time-course of the inactivation are well described by a model similar to that proposed by Heyer et al. (1976. J. Gen. Physiol. 67:703--729) and extended for any value of the diffuse double layer potential and for LA and imipramine. In this model QA, LA, or imipramine are able to permeate through the membrane only when the alamethicin-induced conductance is turned on. The amphipathic compounds then bind to the other membrane surface, changing the transmembrane potential and turning the conductance off. For a given concentration of QA, LA, or imipramine the extent of inactivation depends on two factors: first, the binding characteristics of these compounds to the membrane surface and second, their ability to permeate through the membrane when the alamethicin-induced conductance is turned on. The several possible mechanisms of permeation of the amphipathic molecules tested are discussed.     

Inactivation of the Peptide-Sensitive Channel from the Yeast Mitochondrial Outer Membrane: Properties,Sensitivity to Trypsin and Modulation by a Basic Peptide

The yeast Peptide Sensitive Channel (PSC), a cationic channel of the mitochondrial outer membrane closes with slow kinetics at potentials of either polarity. The properties of this inactivation closely resemble those of the Voltage-Dependent Anion Channel (VDAC) slow kinetics closures. Addition of trypsin to one compartment suppresses the inactivation observed when this compartment is made positive, but does not affect the inactivation observed at potentials of reverse polarity. Both sides of the channel are sensitive. The reduced form of the Mast Cell Degranulating peptide (rMCD) increases the rate of inactivation, but only when the polarity of the compartment to which it is added is positive. The effect is not reversed by washing the peptide out, but is suppressed by trypsin. The peptide can bind to both sides of the membrane. The effect of rMCD on PSC closely resembles that of the ``modulator' on VDAC. The similarities between PSC and VDAC suggest that the former might be a cationic porin of the mitochondrial outer membrane possessing a structure closely related to that of VDAC. Received: 2 February 1996/Revised: 18 October 1996     

Kinetic analysis of pancuronium interaction with sodium channels in squid axon membranes

The interaction of pancuronium with sodium channels was investigated in squid axons. Sodium current turns on normally but turns off more quickly than the control with pancuronium 0.1-1mM present internally; The sodium tail current associated with repolarization exhibits an initial hook and then decays more slowly than the control. Pancuronium induces inactivation after the sodium inactivation has been removed by internal perfusion of pronase. Such pancuronium-induced sodium inactivation follows a single exponential time course, suggesting first order kinetics which represents the interaction of the pancuronium molecule with the open sodium channel. The rate constant of association k with the binding site is independent of the membrane potential ranging from 0 to 80 mV, but increases with increasing internal concentration of pancuronium. However, the rate constant of dissociation l is independent of internal concentration of pancuronium but decreases with increasing the membrane potential. The voltage dependence of l is not affected by changine external sodium concentration, suggesting a current-independent conductance block, The steady-state block depends on the membrane potential, being more pronounced with increasing depolarization, and is accounted for in terms of the voltage dependence of l. A kinetic model, based on the experimental observations and the assumption on binding kinetics of pancuronium with the open sodium channel, successfully simulates many features of sodium current in the presence of pancuronium.     

Alamethicin channels incorporated into frog node of ranvier: calcium-induced inactivation and membrane surface charges

Alamethicin, a peptide antibiotic, partitions into artificial lipid bilayer membranes and into frog myelinated nerve membranes, inducing a voltage-dependent conductance. Discrete changes in conductance representing single-channel events with multiple open states can be detected in either frog node or lipid bilayer membranes. In 120 mM salt solution, the average conductance of a single channel is approximately 600 pS. The channel lifetimes are roughly two times longer in the node membrane than in a phosphatidylethanolamine bilayer at the same membrane potential. With 2 or 20 mM external Ca and internal CsCl, the alamethicin-induced conductance of frog nodal membrane inactivates. Inactivation is abolished by internal EGTA, suggesting that internal accumulation of calcium ions is responsible for the inactivation, through binding of Ca to negative internal surface charges. As a probe for both external and internal surface charges, alamethicin indicates a surface potential difference of approximately -20 to -30 mV, with the inner surface more negative. This surface charge asymmetry is opposite to the surface potential distribution near sodium channels.     

Alamethicin adsorption to a planar lipid bilayer.

The effect of alamethicin and its derivatives on the voltage-dependent capacitance of phosphatidylethanolamine (squalane) membranes was measured using two different methods: lock-in detection and voltage pulse. Alamethicin and its derivatives modulate the voltage-dependent capacitance at voltages lower than the voltage at which alamethicin-induced conductance is detected. The magnitude and sign of this alamethicin-induced capacitance change depends on the aqueous alamethicin concentration and the kind of alamethicin used. Our experimental data can be interpreted as a potential-dependent pseudocapacitance associated with adsorbed alamethicin. Pseudocapacitance is expressed as a function of alamethicin charge, its concentration in the bathing solution and the applied electric field. The theory describes the dependence of the capacitance on applied voltage and alamethicin concentration. When alamethicin is neutral the theory predicts no change of the voltage-dependent capacitance with either sign of applied voltage. Experimental data are consistent with the model in which alamethicin molecules interact with each other while being adsorbed to the membrane surface. The energy of this interaction depends on the alamethicin concentration.     

A binding-site model for calcium channel inactivation that depends on calcium entry

We describe a model for the mechanism by which Ca channel inactivation might depend on calcium entry. Ca is assumed to bind to a site at the internal membrane surface to cause inactivation of Ca channels. We assume that Ca that enters through the membrane accumulates in a submembrane compartment and also make simplifying assumptions about Ca buffering and removal. Our model predicts the results of single- and double-pulse voltage-clamp experiments well. The predicted turn-off of Ca current is non-exponential. The model also predicts that procedures that slow inactivation will increase peak Ca current and suggests that both two-phase turn-off of currents and failure of normalized current to recover to 1.0 in two-pulse experiments may be explained without assuming a voltage-dependent component of inactivation or two populations of Ca channels.     

Receptor-mediated transcytosis of IgA in MDCK cells is via apical recycling endosomes

Classically, the polymeric immunoglobulin receptor and its ligand, IgA, are thought to be sorted from basolateral early endosomes into transcytotic vesicles that directly fuse with the apical plasma membrane. In contrast, we have found that in MDCK cells IgA is delivered from basolateral endosomes to apical endosomes and only then to the apical cell surface. When internalized from the basolateral surface of MDCK cells IgA is found to accumulate under the apical plasma membrane in a compartment that is accessible to two apically added membrane markers: anti-secretory component Fab fragments, and avidin internalized from the biotinylated apical pole of the cell. This accumulation occurs in the presence of apical trypsin, which prevents internalization of the ligand from the apical cell surface. Using a modification of the diaminobenzidine density-shift assay, we estimate that approximately 80% of basolaterally internalized IgA resides in the apical endosomal compartment. In addition, approximately 50% of basolaterally internalized transferrin, a basolateral recycling protein, has access to this apical endosomal compartment and is efficiently recycled back to the basolateral surface. Microtubules are required for the organization of the apical endosomal compartment and it is dispersed in nocodazole-treated cells. Moreover, this compartment is largely inaccessible to fluid-phase markers added to either pole of the cell, and therefore seems analogous to the recycling endosome described in nonpolarized cells. We propose a model in which transcytosis is not a specialized pathway that uses unique transcytotic vesicles, but rather combines portions of pathways used by non- transcytosing molecules.     

Antibody interaction with the amphotericin channel

The monoclonal antibodies against asymmetric channel formed in the lipid bilayer of polyene antibiotic amphotericin B and cholesterol after addition of the antibiotic to the compartment from the cis side of the membrane were obtained. The effect of the antibodies on ion conductance of the channel depends on the distribution of cholesterol in the membrane. When cholesterol was present on both sides of the lipid bilayer, three antibody molecules bound to the channel from the trans side of the membrane, thus markedly increasing the lifetime of the open state of the channel. When cholesterol was present in the cis monolayer only, the antibodies, added to the trans compartment of the cell, reduced the membrane conduction.     

Interaction of nonylguanidine with the sodium channel.

Alkyl and aromatic guanidines interact strongly with the tetrodotoxin (TTX)- receptor site in eel electroplaque membranes, showing competition with TTX. That these guanidines could be useful as highly reversible small molecular weight blockers of Na+ currents is therefore suggested. We have investigated the mechanisms of interaction of one of these derivatives, nonylguanidine, by studying its effects on Na+ currents in squid giant axons using voltage clamp techniques. Although nonylguanidine competed with TTX for binding to eel electroplaque membrane fragments (Ki = 1.8 X 10(-5) M), it reversibly blocked both inward and outward Na+ currents in intact axons only if applied to the interior. In axons with the Na+ inactivation removed by papain nonylguanidine produced a time-dependent block very similar to that reported for strychnine and pancuronium. The reduction of steady-state currents in these axons was also voltage-dependent, with increasing block observed with increasing step depolarization. These results suggest that nonylguanidine binds to a site accessible from the axoplasmic side of the channel, simulating Na+ inactivation in papain-treated axons and competing with the normal inactivation process in untreated axons. The competition between internal nonylguanidine and external TTX may result from perturbation by the positively charged nonylguanidine of the TTX-binding site from within the channel itself.     

Distinct muscarinic mediation of suspected dopaminergic activity in sympathetic ganglions

The competitive neuromuscular blocking agents, gallamine and pancuronium, enhanced the nictitating membrane contraction, in the cat, resulting from muscarine ganglionic transmission. Inhibition of ganglionic muscarinic hyperpolarization, in response to short tetanic bouts of preganglionic cervical sympathetic stimulation, was an associated event and is considered by us to be causally related. The neuroleptic drug, haloperidol, enhanced ganglionic hyperpolarization under similar stimulatory conditions, and reduced the nictitating membrane contraction elicited via ganglionic muscarine pathways, effects opposite to those produced by the skeletal muscle relaxants. Apomorphine reduced both ganglionic hyperpolarization and the ganglionic muscarinic-induced nictitating membrane contractions. The action of gallamine and pancuronium conforms to a speculative cholinergic antagonism at the specific muscarinic receptors, termed Mi, on the ganglionic dopaminergic interneuron. Haloperiodol and apomorphine are anticipated to be exerting distinct antagonistic and agonistic actions, respectively, on prejunctional dopamine receptors of the ganglionic interneuron. Ganglionic slow depolarization mediated through the muscarinic receptors, termed Me, was unaltered by any of the agents studied.     

Characterization, localization, and biosynthesis of acetylcholinesterase in K 562 cells

K 562 cell acetylcholinesterase (AChE), identifiable by active site labeling with radioactive diisopropylfluorophosphate (DFP), showed a Mr around 55,000 in both a crude lysate and a purified sample. The K 562 AChE was reactive with one polyclonal and two monoclonal antibodies produced against human erythrocyte AChE. Subcellular localization, investigated by assay on cell fractions, showed that AChE is membrane bound and that it is located on the cell surface as well as on microsomal and Golgi membranes. Biosynthesis of new enzyme molecules, after inactivation of the constitutive AChE with the irreversible inhibitor DFP, allowed us to follow the kinetics of reappearance in the intracellular compartment and at the cell surface (4 and 8 h, respectively).     

Penetration of Host Cell Membranes by Adenovirus 2

Highly purified human adenovirus type 2 directly penetrated the plasma membranes of KB cells. The process of membrane penetration resulted in the appearance of large numbers of adenovirions free in the cytoplasm of the infected cells. The virions underwent a morphological change as they penetrated the cell surface. Penetration of the plasma membranes and the accompanying alteration in virion morphology was dependent on a function associated with the intact cells, because neither event occurred when purified virions were added to isolated cell membranes. Inactivation of the adenovirions with heat or antibodies before inoculation of the cells reduced the infectivity of the virus population and prevented the appearance of virions free in the cytoplasm. The inactivation of the virions did not significantly reduce the number of virus particles which were found in cell vacuoles and pinocytotic vesicles.     

The orientation of phosphate-dependent glutaminase on the inner membrane of rat renal mitochondria

Phosphate-dependent glutaminase is associated with the inner membrane of rat renal mitochondria. The orientation of this enzyme was characterized by comparing its sensitivity in isolated mitochondria and in mitoplasts to two membrane impermeable inhibitors. Mitoplasts were prepared by repeated swelling of mitochondria in a hypotonic phosphate solution. This procedure released greater than 70% of the adenylate kinase from the intermembrane space, but less than 10 and 25% of the marker activities characteristic of the inner membrane and matrix compartments, respectively. The addition of 20 microM p-chloromercuriphenylsulfonate (pCMPS) caused a rapid inactivation of the purified glutaminase. In contrast, the glutaminase contained in isolated mitochondria and mitoplasts was only slightly affected by the addition of up to 2 mM pCMPS. Similarly, the activity in mitochondria and mitoplasts was not inhibited by the addition of an excess of inactivating Fab antibodies. However, a similar extent of inactivation occurred when either membrane fraction was incubated with concentrations of octylglucoside greater than 0.35%. Mitochondria were also treated with increasing concentrations of digitonin. At 0.4 mg digitonin/mg protein, all of the adenylate kinase was released but the glutaminase activity was either slightly inhibited or unaffected by the addition of pCMPS or the Fab antibodies, respectively. These studies establish that the glutaminase is localized on the inner surface of the inner membrane. Therefore, mitochondrial catabolism of glutamine must occur only within the matrix compartment.     

The deubiquitinating enzyme USP10 regulates the endocytic recycling of CFTR in airway epithelial cells

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a cyclic AMP-regulated chloride channel that plays an important role in regulating the volume of the lung airway surface liquid, and thereby mucociliary clearance and elimination of pathogens from the lung. In epithelial cells, cell surface CFTR abundance is determined in part by regulating both CFTR endocytosis from the apical plasma membrane and recycling back to the plasma membrane. We recently reported, using an activity-based chemical screen to identify active deubiquitinating enzymes (DUBs) in human airway epithelial cells, that Ubiquitin Specific Protease-10 (USP10) is located and active in the early endosomal compartment and regulates the deubiquitination of CFTR and thereby promotes its endocytic recycling. siRNA-mediated knockdown of USP10 increased the multi-ubiquitination and lysosomal degradation of CFTR and decreased the endocytic recycling and the half-life of CFTR in the apical membrane, as well as CFTR-mediated chloride secretion. Over-expression of wild-type USP10 reduced CFTR multi-ubiquitination and degradation, while over-expression of a dominant-negative USP10 promoted increased multi-ubiquitination and lysosomal degradation of CFTR. In the current study, we show localization and activity of USP10 in the early endosomal compartment of primary bronchial epithelial cells, as well as an interaction between CFTR and USP10 in this compartment. These studies demonstrate a novel function for USP10 in facilitating the deubiquitination of CFTR in early endosomes, thereby enhancing the endocytic recycling and cell surface expression of CFTR.     

Ligand-independent internalization and recycling of the insulin receptor. Effects of chronic treatment of 3T3-C2 fibroblasts with insulin and dexamethasone.

Upon binding insulin at the plasma membrane, the insulin receptor internalizes into the endosomal compartment of the cell with a half-time of approximately 10 min. Our earlier work demonstrated that receptor inactivation (loss of insulin binding capacity) is a regulated process. Long term treatment of cultured cells with insulin or the glucocorticoid dexamethasone increases or decreases, respectively, the rate constant for insulin receptor inactivation (Knutson, V. P., Ronnett, G. V., and Lane, M. D. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2822-2826). In these studies, monolayer cultures of 3T3-C2 fibroblasts were chronically treated with insulin or dexamethasone. Subsequently, the surface receptors were labeled with the photoactivatable cross-linking agent 125I-labeled 2-(p-azidosalicylamido)ethyl-1,3'- dithiopropionate -insulin. Following equilibration of the radiolabeled receptor between the plasma membrane and internal pools, the steady-state rate constant for receptor recycling was determined by quantitating the rate at which internal radiolabeled receptor was inserted into the plasma membrane. The steady-state rate constant for this recycling process was the same in control, insulin-treated, or steroid-treated cells (t1/2 = 2h). In contrast, the rate constant for receptor internalization was regulated; the half-times were 10 h for control cells, 5 h for insulin-treated cells, and 19 h for dexamethasone-treated cells. These changes in rate constants for internalization and inactivation lead to changes in the relative numbers of receptor molecules undergoing recycling versus inactivation. Therefore, whereas the recycling of the insulin receptor is not a regulated process, the internalization of surface receptor in the absence of bound ligand is a metabolically controlled step in receptor processing.     

Blocking of the beta-latrotoxin channels in bimolecular lipid membranes by antibodies

Asymmetric ion channels are formed in a bimolecular lipid membrane by beta-latrotoxin (LT) introduced to one (cis) side of the membrane. LT-specific antibodies added to the opposite (trans) side of the membrane block the current through the LT channels when a negative potential is applied to the cis side, no blockade is observed at positive potentials. LT-specific antibodies do not block the channel current when added to the cis compartment after removal of LT. LT-unspecific immunoglobulins have no influence on LT channel conductance.     

Amiloride-sensitive trypsinization of apical sodium channels. Analysis of hormonal regulation of sodium transport in toad bladder

Incubation of the mucosal surface of the toad urinary bladder with trypsin (1 mg/ml) irreversibly decreased the short-circuit current to 50% of the initial value. This decrease was accompanied by a proportionate decrease in apical Na permeability, estimated from the change in amiloride-sensitive resistance in depolarized preparations. In contrast, the paracellular resistance was unaffected by trypsinization. Amiloride, a specific blocker of the apical Na channels, prevented inactivation by trypsin. Inhibition of Na transport by substitution of mucosal Na, however, had no effect on the response to trypsin. Trypsinization of the apical membrane was also used to study regulation of Na transport by anti-diuretic hormone (ADH) and aldosterone. Prior exposure of the apical surface to trypsin did not reduce the response to ADH, which indicates that the ADH-induced Na channels were inaccessible to trypsin before addition of the hormone. On the other hand, stimulation of short-circuit current by aldosterone or pyruvate (added to substrate-depleted, aldosterone-repleted bladders) was substantially reduced by prior trypsinization of the apical surface. Thus, the increase in apical Na permeability elicited by aldosterone or substrate involves activation of Na channels that are continuously present in the apical membrane in nonconductive but trypsin-sensitive forms.     

Voltage-dependent lipid flip-flop induced by alamethicin.

Alamethicin appears to allow voltage-dependent lipid exchange ("flip-flop") between leaflets of a planar bilayer. In membranes with one leaflet of phosphatidyl serine and one of phosphatidyl ethanolamine, the shape of the nonactin current-voltage curve accurately reports the difference in surface potential between the two sides of the membrane. The surface potential is itself a good measure of membrane asymmetry. Alamethicin added to the bathing solutions of an asymmetric membrane does not per se reduce the membrane asymmetry, but turning on the alamethicin conductance by application of a voltage pulse does. Immediately after application of a voltage pulse, large enough to turn on the alamethicin conductance, the asymmetry of the nonactin-K+ current voltage curve decreases, in some cases, nearly to zero. During the pulse, the alamethicin conductance activates if a decrease in surface potential favors turn-on of the alamethicin conductance or inactivates if a decrease in surface potential favors turn-off of the alamethicin conductance. After the pulse, the nonactin-K+ asymmetry returns to its original value if the alamethicin conductance is not turned on. The time-course of this return allows an estimate of the diffusion constant of lipid in the planar bilayer. The value obtained is 5.1 x 10(-8) cm2/s.     

Synaptic-like Microvesicles of Neuroendocrine Cells Originate from a Novel Compartment That Is Continuous with the Plasma Membrane and Devoid of Transferrin Receptor

We have characterized the compartment from which synaptic-like microvesicles (SLMVs), the neuroendocrine counterpart of neuronal synaptic vesicles, originate. For this purpose we have exploited the previous observation that newly synthesized synaptophysin, a membrane marker of synaptic vesicles and SLMVs, is delivered to the latter organelles via the plasma membrane and an internal compartment. Specifically, synaptophysin was labeled by cell surface biotinylation of unstimulated PC12 cells at 18°C, a condition which blocked the appearance of biotinylated synaptophysin in SLMVs and in which there appeared to be no significant exocytosis of SLMVs. The majority of synaptophysin labeled at 18°C with the membraneimpermeant, cleavable sulfo-NHS-SS–biotin was still accessible to extracellularly added MesNa, a 150-D membrane-impermeant thiol-reducing agent, but not to the 68,000-D protein avidin. The SLMVs generated upon reversal of the temperature to 37°C originated exclusively from the membranes containing the MesNaaccessible rather than the MesNa-protected population of synaptophysin molecules. Biogenesis of SLMVs from MesNa-accessible membranes was also observed after a short (2 min) biotinylation of synaptophysin at 37°C followed by chase. In contrast to synaptophysin, transferrin receptor biotinylated at 18° or 37°C became rapidly inaccessible to MesNa. Immunofluorescence and immunogold electron microscopy of PC12 cells revealed, in addition to the previously described perinuclear endosome in which synaptophysin and transferrin receptor are colocalized, a sub-plasmalemmal tubulocisternal membrane system distinct from caveolin-positive caveolae that contained synaptophysin but little, if any, transferrin receptor. The latter synaptophysin was selectively visualized upon digitonin permeabilization and quantitatively extracted, despite paraformaldehyde fixation, by Triton X-100. Synaptophysin biotinylated at 18°C was present in these subplasmalemmal membranes. We conclude that SLMVs originate from a novel compartment that is connected to the plasma membrane via a narrow membrane continuity and lacks transferrin receptor.     

Modification of sodium channel currents by lanthanum and lanthanide ions in human heart cells

I examined the effects of 100 microM extracellular lanthanum and lanthanide ions on the fast transmembrane sodium channel currents of human heart cell segments. The experiments were conducted under control of the transmembrane electrical and chemical gradients. Lanthanum and lanthanide ion exposure decreased the amplitude and increased the inactivation time constant of the sodium current. Only a transient increase occurred for the activation time constant of the sodium current. The dependence of peak sodium current on excitatory and holding potentials (steady-state activation and inactivation curves, respectively) was transiently shifted to less negative potentials during the first 3 min of exposure, as if these cations were momentarily neutralizing the effective negative charges at the extracellular side of the membrane. The curves then returned to their original position and only the inactivation curves continued shifting progressively towards a limit at more negative membrane potentials. Membrane capacitance was always reduced and this may explain these late effects in terms of changes in membrane dielectric properties and free and bound charges, in addition to traditional screening and binding concepts. These effects were related to the electronic structure of these ions.