Microstructured apertures in planar glass substrates for ion channel research

We have developed planar glass chip devices for patch clamp recording. Glass has several key advantages as a substrate for planar patch clamp devices. It is a good dielectric, is well-known to interact strongly with cell membranes and is also a relatively in-expensive material. In addition, it is optically neutral. However, microstructuring processes for glass are less well established than those for silicon-based substrates. We have used ion-track etching techniques to produce micron-sized apertures into borosilicate and quartz-glass coverslips. These apertures, which can be easily produced in arrays, have been used for high resolution recording of single ion channels as well as for whole-cell current recordings from mammalian cell lines. An additional attractive application that is greatly facilitated by the combination of planar geometry with the optical neutrality of the substrate is single-molecule fluorescence recording with simultaneous single-channel measurements.

[Cell wall components of gramicidin S resistant Staphylococcus aureus]

Comparative study of two staphylococcus aureus 209P strains--resistant and susceptible to gramicidin S demonstrated that peptidoglycanes of two strains differ by ratio glycine/serine at peptide bridges. Besides peptidoglycanes significantly differ by amidation of alfa-carboxyles of glutamic acid in muropeptide. This peptidoglycane modification of resistant cells along with enhanced content of etherized D-alanine in teichoic acid provides lower negative charge of cell wall components. It may influence the cell wall ability to react with positively charged gramicidin molecules. It was shown that isolated cell walls and peptidoglycane of resistant cells binds significantly less gramicidin than cell walls and peptodoglyce of susceptable cells. Simultaneous determination of gramicidin binding by intact S. aureus cells and their killing revealed that lower ability of resistant cells to bind gramicidin is significant but not critical factor of gramicidin resistance.

Synthesis and biological evaluation of novel turn-modified gramicidin S analogues

The synthesis of novel gramicidin S analogues having additional functionalities in the turn region by employing a biomimetic approach is described. The preservation of beta-sheet character in all analogues was established by NMR and the biological activity was evaluated.

Extra amino group-containing gramicidin S analogs possessing outer membrane-permeabilizing activity

Novel (2S,4R)- and (2S,4S)-4-aminoproline residue-containing analogs of the cyclic decapeptide antibiotic gramicidin S were synthesized, which exhibited marked permeabilizing activity on the outer membrane of gram-negative bacteria.

Transport of L-arginine and nitric oxide formation in human platelets

The results of the present study show that human platelets take up l-arginine by two transport systems which are compatible with the systems y+ and y+L. These Na+independent transporters have been distinguished by treating platelets with N-ethylmaleimide that blocks selectively system y+. System y+, that accounts for 30-40% of the total transport, is characterized by low affinity for l-arginine, is unaffected by l-leucine, is sensitive to changes of membrane potential and to trans-stimulation. The other component of l-arginine transport identified with the system y+L (approximately 60-70% of the total flux) shows high affinity for l-arginine, is insensitive to N-ethylmaleimide treatment, unaffected by changes in membrane potential, sensitive to trans-stimulation and inhibited by l-leucine in the presence of Na+. Moreover a strict correlation between l-arginine transport and nitric oxide (NO) production in whole cells was found. N-ethylmaleimide and l-leucine decreased NO production as well as cGMP elevation, and the effect on NO and cGMP were closely related. It is likely that the l-arginine transport systems y+ and y+L are both involved in supplying substrate for NO production and regulation in human platelets.

A fluorimetric microplate assay for detection and quantitation of toxins causing paralytic shellfish poisoning

Paralytic shellfish poisoning is one of the most severe forms of food poisoning. The toxins responsible for this type of poisoning are metabolic products of dinoflagellates, which block neuronal transmission by binding to the voltage-gated Na(+) channel. Accumulation of paralytic toxins in shellfish is an unpredictable phenomenon that necessitates the implementation of a widespread and thorough monitoring program for mollusk toxicity. All of these programs require periodical collection and analysis of a wide range of shellfish. Therefore, development of accurate analytical protocols for the rapid determination of toxicity levels would streamline this process. Our laboratory has developed a fluorimetric microplate bioassay that rapidly and specifically determines the presence of paralytic shellfish toxins in many seafood samples. This method is based on the pharmacological activity of toxins and involves several steps: (i) Incubation of excitable cells in 96 well microtiter plates with the fluorescent dye, bis-oxonol, the distribution of which across the membrane is potential-dependent. (ii) Cell depolarization with veratridine, a sodium channel-activating toxin. (iii) Dose-dependent inhibition of depolarization with saxitoxin or natural samples containing paralytic shellfish toxins. Measuring toxin-induced changes in membrane potential allowed for quantification and estimation of the toxic potency of the samples. This new approach offers significant advantages over classical methods and can be easily automated.

Differential sensitivity of atrial and ventricular K(ATP) channels to metabolic inhibition

OBJECTIVE

The aim is to compare the activation of ATP-sensitive potassium channels (K(ATP) channels) in intact and metabolically impaired atrial and ventricular myocytes.

METHODS

The K(ATP) channel current is measured by whole cell and gramicidin-perforated patch clamp recordings in 164 cultured neonate rat cardiomyocytes.

RESULTS

In whole cell recordings with 84 micromol/l ADP in pipette, spontaneous activity is significantly higher in atrium than ventricle, and EC(50) for the K(ATP) channel opener diazoxide is 0.13 micromol/l (atrium) versus 3.1 micromol/l (ventricle). With an ATP-regenerating system in pipette, EC(50) for diazoxide is 19.7 micromol/l (atrium) versus 54.9 micromol/l (ventricle). In gramicidin-perforated patch recordings, atrial myocytes respond significantly to 100 nmol/l of the mitochondrial protonophore CCCP, while ventricular myocytes do not. EC(50) for diazoxide is 129 micromol/l (atrium) versus >2500 micromol/l (ventricle) for myocytes exposed to CCCP, and 676 versus >2500 micromol/l, respectively, without CCCP.

CONCLUSIONS

(1) K(ATP) channels are significantly more sensitive to metabolic inhibition in atrial than ventricular myocytes. (2) Sensitivity of atrium versus ventricle to the channel opener diazoxide increases from 3:1 to > or = 24:1 with ADP or metabolic inhibition. If extended to intact hearts, the results would predict a higher atrial sensitivity to ischemia, and a high sensitivity of the ischemic atrium to K(ATP) channel openers.

Fourier transform infrared spectroscopic study of the interactions of a strongly antimicrobial but weakly hemolytic analogue of gramicidin S with lipid micelles and lipid bilayer membranes

Cyclo[VKLdKVdYPLKVKLdYP] (GS14dK(4)), a synthetic tetradecameric ring-size analogue of the naturally occurring antimicrobial peptide gramicidin S (GS), retains the strong antimicrobial activity of GS but is 15-20 times less hemolytic. To characterize its interaction with lipid membranes and to understand the molecular basis of its capacity to lyse bacterial cells, in preference to erythrocytes, we have investigated the interactions of GS14dK(4) with detergent micelles and with lipid bilayer model membranes by Fourier transform infrared spectroscopy and compared our results with those of a similar study of GS [Lewis, R. N. A. H., et al. (1999) Biochemistry 38, 15193-15203]. In both aqueous and organic solvent solutions, GS14dK(4) adopts a beta-sheet conformation that is somewhat distorted and more sensitive to the polarity of its environment than GS. Like GS, GS14dK(4) is completely or partially excluded from gel-state lipid bilayers but interacts strongly with liquid-crystalline lipid bilayers and detergent micelle, and interacts more strongly with more fluid liquid-crystalline lipid systems. However, its interactions are more strongly influenced by membrane lipid order and fluidity, and unlike GS, it is essentially excluded from cholesterol-containing phospholipid bilayers. Also, GS14dK(4) is excluded from cationic lipid bilayers, but partitions more strongly and/or penetrates more deeply into anionic lipid bilayers than into those composed of either zwitterionic or nonionic lipids. Anionic lipids also facilitate GS14dK(4) interactions with multicomponent lipid bilayers which are predominantly zwitterionic or nonionic. Although GS14dK(4) generally penetrates and/or partitions into zwitterionic or uncharged lipid bilayers less strongly than does GS, its greater size and altered distribution of positive charges make it intrinsically more perturbing with regard to membrane organization once associated with lipid bilayers. This fact, combined with its relatively strong interactions with anionic phospholipids, may explain why GS14dK(4) retains relatively high antimicrobial activity. However, its low hemolytic activity is probably largely attributable to its low propensity to penetrate and/or partition into cholesterol-containing zwitterionic lipid membranes.

Electrophysiological response of cultured trabecular meshwork cells to synthetic ion channels

The response of living cells of the trabecular meshwork to synthetic ion channels is described. The THF-gramicidin hybrids THF-gram and THF-gram-TBDPS as well as a linked gA-TBDPS and gramicidin A were applied to cultured ocular trabecular meshwork cells. THF-gram application (minimal concentration, 10(-8) M; saturation, 10(-7) M) led to an additional conductance which displayed characteristics of weak Eisenman-I-selective cation channels, no cell destruction, an asymmetric change of the inward/outward currents, and higher current densities using Cs(+) as charge carrier compared to Na(+) and K(+). Linked-gA-TBDPS showed at 10(-12) M increases of the membrane conductance comparable to gA at 10(-7) M and a much faster response of the cells. Thus, THF-gramicidin hybrids form a basis for the use of synthetic ion channels in biological systems, which eventually may lead to new therapeutic approaches.

Nonspecific depolarization of the plasma membrane potential induces cytoskeletal modifications of bovine corneal endothelial cells in culture

Modifications in the cell membrane potential have been suggested to affect signaling mechanisms participating in diverse cellular processes, many of which involve structural cellular alterations. In order to contribute some evidence in this respect, we explored the effects of several depolarizing procedures on the structure and monolayer organization of bovine corneal endothelial cells in culture. Visually confluent cell monolayers were incubated with or without the depolarizing agent, either in a saline solution or in culture medium for up to 30 min. Membrane potential was monitored by fluorescence microscopy using oxonol V. Fluorescent probes were employed for F-actin, microtubules, and vinculin. Depolarization of the plasma membrane, achieved via the incorporation of gramicidin D into confluent endothelial cells or by modifications of the extracellular saline composition, provoked an increment of oxonol fluorescence and changes in cell morphology, consisting mainly of modifications in the cytoskeletal organization. In some areas, noticeable intercellular spaces appear. The cytoskeleton modifications mainly consist of a marked redistribution of F-actin and microtubules, with accompanying changes in vinculin localization. The results suggest that the depolarization of the plasma membrane potential may participate in mechanisms involved in cytoskeleton organization and monolayer continuity in corneal endothelial cells in culture.

In depolarized and glucose-deprived neurons, Na+ influx reverses plasmalemmal K+-dependent and K+-independent Na+/Ca2+ exchangers and contributes to NMDA excitotoxicity

Cerebellar granule cells (CGCs) express K+-dependent (NCKX) and K+-independent (NCX) plasmalemmal Na+/Ca2+ exchangers which, under plasma membrane-depolarizing conditions and high cytosolic [Na+], may reverse and mediate potentially toxic Ca2+ influx. To examine this possibility, we inhibited NCX or NCKX with KB-R7943 or K+-free medium, respectively, and studied how gramicidin affects cytosolic [Ca2+] and 45Ca2+ accumulation. Gramicidin forms pores permeable to alkali cations but not Ca2+. Therefore, gramicidin-induced Ca2+ influx is indirect; it results from fluxes of monovalent cations. In the presence of Na+, but not Li+ or Cs+, gramicidin induced Ca2+ influx that was inhibited by simultaneous application of KB-R7943 and K+-free medium. The data indicate that gramicidin-induced Na+ influx reverses NCX and NCKX. To test the role of NCX and/or NCKX in excitotoxicity, we studied how NMDA affects the viability of glucose-deprived and depolarized CGCs. To assure depolarization of the plasma membrane, we inhibited Na+,K+-ATPase with ouabain. Although inhibition of NCX or NCKX reversal failed to significantly limit 45Ca2+ accumulation and excitotoxicity, simultaneously inhibiting NCX and NCKX reversal was neuroprotective and significantly decreased NMDA-induced 45Ca2+ accumulation. Our data suggest that NMDA-induced Na+ influx reverses NCX and NCKX and leads to the death of depolarized and glucose-deprived neurons.

The effects of ring-size analogs of the antimicrobial peptide gramicidin S on phospholipid bilayer model membranes and on the growth of Acholeplasma laidlawii B

We have examined the effects of three ring-size analogs of the cyclic beta-sheet antimicrobial peptide gramicidin S (GS) on the thermotropic phase behavior and permeability of phospholipid model membranes and on the growth of the cell wall-less Gram-positive bacteria Acholeplasma laidlawii B. These three analogs have ring sizes of 10 (GS10), 12 (GS12) or 14 (GS14) amino acids, respectively. Our high-sensitivity differential scanning calorimetric studies indicate that all three of these GS analogs perturb the gel/liquid-crystalline phase transition of zwitterionic phosphatidylcholine (PtdCho) vesicles to a greater extent than of zwitterionic phosphatidylethanolamine (PtdEtn) or of anionic phosphatidylglycerol (PtdGro) vesicles, in contrast to GS itself, which interacts more strongly with PtdGro than with PtdCho and PtdEtn bilayers. However, the relative potency of the perturbation of phospholipid phase behavior varies markedly between the three peptides, generally decreasing in the order GS14 > GS10 > GS12. Similarly, these three GS ring-size analogs also differ considerably in their ability to cause fluorescence dye leakage from phospholipid vesicles, with the potency of permeabilization also generally decreasing in the order GS14 > GS10 > GS12. Finally, these GS ring-size analogs also differentially inhibit the growth of A. laidlawii with growth inhibition also decreasing in the order GS14 > GS10 > GS12. These results indicate that the relative potencies of GS and its ring-size analogs in perturbing the organization and increasing the permeability of phospholipid bilayer model membranes, and of inhibiting the growth of A. laidlawii B cells, are at least qualitatively correlated, and provide further support for the hypothesis that the primary target of these antimicrobial peptides is the lipid bilayer of the bacterial membrane. The very high antimicrobial activity of GS14 against the cell wall-less bacteria A. laidlawii as compared to various conventional bacteria confirms our earlier suggestion that the avid binding of this peptide to the bacterial cell wall is primarily responsible for its reduced antimicrobial activity against such organisms. The relative magnitude of the effects of GS itself, and of the three ring-size GS analogs, on phospholipid bilayer organization and cell growth correlate relatively well with the effective hydrophobicities and amphiphilicities of these peptides but less well with their relative charge density, intrinsic hydrophobicities or conformational flexibilities. Nevertheless, all of these parameters, as well as others, may influence the antimicrobial potency and hemolytic activity of GS analogs.

Morphological characteristics by SEM observations and regulatory volume decrease (RVD) of tentacular nematocytes isolated by heat dissociation from Aiptasia diaphana (Cnidaria: Anthozoa)

This paper examines the morphological characteristics and response to hyposmotic shock of nematocytes isolated by heat dissociation at 45 degrees C for 20 min from the tentacles of Aiptasia diaphana, an anthozoan living in the brackish water of Lake Faro (Messina, Italy). Morphological characteristics were evaluated by scanning electron microscopy (SEM) observations and cytological test, functional characteristics by exposure to 35% hyposmotic stress. 81% of the population of isolated nematocytes had ciliary protrusions. Microbasic-mastigophore and amastigophore nematocytes had a hair bundle at the crown shaped apex composed of ciliary protrusions of different heights and diameters. In basitrichs, instead, a single ciliary protrusion was observed. Following exposure to 35% hyposmotic shock, nematocytes isolated by heat dissociation did not show RVD, while, following treatment with 1 microM and 2 microM gramicidin-S, activation of volume regulation in conditions of hyposmoticity was observed. The effect of gramicidin was concentration-dependent and confirmed the relevant role of conductance to K+ in volume regulation, as previously described (17) on nematocytes isolated by chemical methods.

Exploring relationships between mimic configuration, peptide conformation and biological activity in indolizidin-2-one amino acid analogs of gramicidin S

Indolizidin-2-one amino acids (I2aas, 6S- and 6R-1) possessing 6S- and 6R-ring-fusion stereochemistry were introduced into the antimicrobial peptide gramicidin S (GS) to explore the relationships between configuration, peptide conformation and biological activity. Solution-phase and solid-phase techniques were used to synthesize three analogs with I2aa residues in place of the d-Phe-Pro residues at the turn regions of GS: [(6S)-I2aa4-5,4'-5']GS (2), [Lys2,2',(6S)-I2aa4-5,4'-5']GS (3) and [(6R)-I2aa4-5,4'-5']GS (4). Although conformational analysis of [I2aa4-5,4'-5']GS analogs 2-4 indicated that both ring-fusion stereoisomers of I2aa gave peptides with CD and NMR spectral data characteristic of GS, the (6S)-I2aa analogs 2 and 3 exhibited more intense CD curve shapes, as well as greater numbers of nonsequential NOE between opposing Val and Leu residues, relative to the (6R)-I2aa analog 4, suggesting a greater propensity for the (6S)-diastereomer to adopt the beta-turn/antiparallel beta-pleated sheet conformation. In measurements of antibacterial and antifungal activity, the (6S)-I2aa analog 2 exhibited significantly better potency than the (6R)-I2aa diastereomer 4. Relative to GS, [(6S)-I2aa4-5,4'-5']GS (2) exhibited usually 1/2 to 1/4 antimicrobial activity as well as 1/4 hemolytic activity. In certain cases, antimicrobial and hemolytic activities of GS were shown to be dissociated through modification at the peptide turn regions with the (6S)-I2aa diastereomer. The synthesis and evaluation of GS analogs 2-4 has furnished new insight into the importance of ring-fusion stereochemistry for turn mimicry by indolizidin-2-one amino acids as well as novel antimicrobial peptides.

Continuum electrostatics fails to describe ion permeation in the gramicidin channel

We investigate the validity of continuum electrostatics in the gramicidin A channel using a recently determined high-resolution structure. The potential and electric field acting on ions in and around the channel are computed by solving Poisson's equation. These are then used in Brownian dynamics simulations to obtain concentration profiles and the current passing through the channel. We show that regardless of the effective dielectric constant used for water in the channel or the channel protein, it is not possible to reproduce all the experimental data on gramicidin A; thus, continuum electrostatics cannot provide a valid framework for the description of ion dynamics in gramicidin channels. Using experimental data and molecular dynamics simulations as guides, we have constructed potential energy profiles that can satisfactorily describe the available physiological data. These profiles provide useful benchmarks for future potential of mean force calculations of permeating ions from molecular dynamics simulations of gramicidin A. They also offer a convenient starting point for studying structure-function relationships in modified gramicidin channels.

Synthesis and study of a gramicidin B mutant possessing a ditryptophan crosslink

Recent studies of peptide dimers linked by Trp-Trp (ditryptophan) crosslinks suggest that the crosslinks can reinforce antiparallel beta-structure. Depending on environment, gramicidins A, B and C form either helical ion channels with parallel beta-structure or non-functional pores with antiparallel beta-structure. In the channel conformation of the gramicidins Trp9 and Trp15 are close in space, but in the pore conformation Trp9 and Trp15 are far apart. We hypothesized that a ditryptophan crosslink between Trp9 and Trp15 could pre-organize gramicidin in an active conformation. To test the potential for preorganization, an intramolecular ditryptophan crosslink was formed between Trp9 and Trp15 in a W13F mutant of gramicidin B. Photooxidative conditions were shown to generate ditryptophan crosslinks in low yields. While not preparatively useful, photooxidative tryptophan crosslinking may have implications for protein aging processes like cataract formation. The ditryptophan crosslink in the gramicidin B mutant substantially lowered the antibiotic activity of the gramicidin B mutant, unlike the ditryptophan crosslink in the antibiotic X-indolicidin. The biaryl chromophore generated diagnostic Cotton effects in the CD spectrum that revealed the absolute stereochemistry of the biaryl chromophore, but the biaryl chromophore obscured diagnostic features below 220 nm. However, changes in peptide conformation were reflected in changes in the biaryl region of the CD spectrum above 240 nm.

Effects of volatile anesthetic on channel structure of gramicidin A

Volatile anesthetic agent, 1-chloro-1,2,2-trifluorocyclobutane (F3), was found to alter gramicidin A channel function by enhancing Na(+) transport (. Biophys. J. 77:739-746). Whether this functional change is associated with structural alternation is evaluated by circular dichroism and nuclear magnetic resonance spectroscopy. The circular dichroism and nuclear magnetic resonance results indicate that at low millimolar concentrations, 1-chloro-1,2,2-trifluorocyclobutane causes minimal changes in gramicidin A channel structure in sodium dodecyl sulfate micelles. All hydrogen bonds between channel backbones are well maintained in the presence of 1-chloro-1,2,2-trifluorocyclobutane, and the channel structure is stable. The finding supports the notion that low affinity drugs such as volatile anesthetics and alcohols can cause significant changes in protein function without necessarily producing associated changes in protein structure. To understand the molecular mechanism of general anesthesia, it is important to recognize that in addition to structural changes, other protein properties, including dynamic characteristics of channel motions, may also be of functional significance.

Depolarizing glycine responses in Cajal-Retzius cells of neonatal rat cerebral cortex

We investigated the properties of glycine-induced responses in Cajal-Retzius cells, a neuronal cell type essential for the establishment of neocortical lamination. Whole-cell and gramicidin-perforated patch-clamp recordings were performed on visually identified Cajal-Retzius cells in tangential slices from neonatal rat cortex (postnatal days 0-3). With a pipette Cl(-) concentration of 50 mM, bath application of 1 mM glycine induced a membrane depolarization of 32.8+/-7.4 mV and a massive decrease in membrane resistance by 88+/-1.4%. The membrane depolarization was abolished in the presence of the glycinergic antagonists strychnine (30 microM) and phenylbenzene-omega-phosphono-alpha-amino acid (100 microM), while the GABA(A) receptor antagonist bicuculline (100 microM) and the glutamatergic antagonist (+/-)-2-amino-5-phosphonopentatonic acid (60 microM) were without effect, suggesting that the glycine-induced membrane responses were mediated exclusively by the strychnine-sensitive glycine receptor. The EC(50) for activation of glycine receptors was 0.54 mM, 1.62 mM and 2.41 mM, for the glycinergic agonists glycine, beta-alanine and taurine, respectively. Since the reversal potential of the glycine-induced currents showed a strong dependency on the intracellular chloride concentration and was virtually unaffected under HCO(3)(-)-free conditions, the activation of glycine receptors was probably linked to Cl(-) fluxes with little contribution of HCO(3)(-) ions. Perforated patch recordings from Cajal-Retzius cells demonstrated that glycine elicited depolarizing responses mediated by Cl(-) currents which reversed at -41+/-3.7 mV. In summary, from these results we suggest that Cajal-Retzius cells of the neonatal rat cerebral cortex express functional strychnine-sensitive glycine receptors that mediate depolarizing membrane responses via Cl(-) efflux.

The role of Trp side chains in tuning single proton conduction through gramicidin channels

We present an extensive set of measurements of proton conduction through gramicidin A (gA), B (gB), and M (gM) homodimer channels which have 4, 3, or 0 Trp residues at each end of the channel, respectively. In gA we find a shoulder separating two domains of conductance increasing with concentration, confirming the results of Eisenman, G., B. Enos, J. Hagglund, and J. Sandblom. 1980. Ann. NY. Acad. Sci. 339:8-20. In gB, the shoulder is shifted by approximately 1/2 pH unit to higher H(+) concentrations and is very sharply defined. No shoulder appears in the gM data, but an associated transition from sublinear to superlinear I-V values occurs at a 100-fold higher [H(+)] in gM than in gA. The data in the low concentration domain are analyzed using a configuration space model of single-proton conduction, assuming that the difference in the proton potential of mean force (PMF) between gA and its analogs is constant, similar to the results of Anderson, D., R. B. Shirts, T. A. Cross, and D. D. Busath. 2001. Biophys. J. 81:1255-1264. Our results suggest that the average amplitudes of the calculated proton PMFs are nearly correct, but that the water reorientation barrier calculated for gA by molecular dynamics using the PM6 water model (Pomès, R., and B. Roux. 1997. Biophys. J. 72:246a) must be reduced in amplitude by 1.5 kcal/mol or more, and is not rate-limiting for gA.

NADH oxidation and NAD+ reduction catalysed by tightly coupled inside-out vesicles from Paracoccus denitrificans

Tightly coupled inside-out vesicles were prepared from Paracoccus denitrificans cells (SPP, sub-Paracoccus particles) and characterized kinetically. The rate of NADH oxidation, catalysed by SPP, increases 6-8 times on addition of gramicidin. The vesicles are capable of catalysing Delta micro H+-dependent reverse electron transfer from quinol to NAD+. The kinetic parameters of the NADH-oxidase and the reverse electron transfer carried out by membrane-bound P. denitrificans complex I were estimated and compared with those of the mitochondrial enzyme. The data demonstrate that catalytic properties of the dinucleotide-binding site of the bacterial and mitochondrial complex I are almost identical, pointing out similar organization of the site in mammals and P. denitrificans. Inhibition of the bacterial complex I by a specific inhibitor of Q reduction, rotenone, is very different from that of the mitochondrial enzyme. The inhibitor is capable of suppressing the NADH oxidation reaction only at micromolar concentrations, while the activity of mitochondrial enzyme is suppressed by nanomolar concentrations of rotenone. In contrast to the mitochondrial enzyme, rotenone, even at concentrations as high as 10 micro m, does not inhibit the reverse, Delta micro H+-dependent NAD+-reductase reaction on SPP.

A "minimal" sodium channel construct consisting of ligated S5-P-S6 segments forms a toxin-activatable ionophore

The large size (six membrane-spanning repeats in each of four domains) and asymmetric architecture of the voltage-dependent Na+ channel has hindered determination of its structure. With the goal of determining the minimum structure of the Na+ channel permeation pathway, we created two stable cell lines expressing the voltage-dependent rat skeletal muscle Na+ channel (micro1) with a polyhistidine tag on the C terminus (muHis) and pore-only micro1 (muPore) channels with S1-S4 in all domains removed. Both constructs were recognized by a Na+ channel-specific antibody on a Western blot. muHis channels exhibited the same functional properties as wild-type micro1. In contrast, muPore channels did not conduct Na+ currents nor did they bind [3H]saxitoxin. Veratridine caused 40 and 54% cell death in muHis- and muPore-expressing cells, respectively. However, veratridine-induced cell death could only be blocked by tetrodotoxin in cells expressing muHis, but not muPore. Furthermore, using a fluorescent Na+ indicator, we measured changes in intracellular Na+ induced by veratridine and a brevotoxin analogue, pumiliotoxin. When calibrated to the maximum signal after addition of gramicidin, the maximal percent increases in fluorescence (deltaF) were 35 and 31% in cells expressing muHis and muPore, respectively. Moreover, in the presence of 1 microm tetrodotoxin, deltaF decreased significantly to 10% in muHis- but not in muPore-expressing cells (43%). In conclusion, S5-P-S6 segments of micro1 channels form a toxin-activable ionophore but do not reconstitute the Na+ channel permeation pathway with full fidelity.

Modulation of concentration fluctuations in phase-separated lipid membranes by polypeptide insertion

The lateral membrane organization and phase behavior of the binary lipid mixture DMPC (1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine) - DSPC (1,2-distearoyl-sn-glycero-3-phosphatidylcholine) without and with incorporated gramicidin D (GD) as a model biomembrane polypeptide was studied by small-angle neutron scattering, Fourier-transform infrared spectroscopy, and by two-photon excitation fluorescence microscopy on giant unilamellar vesicles. The small-angle neutron scattering method allows the detection of concentration fluctuations in the range from 1 to 200 nm. Fluorescence microscopy was used for direct visualization of the lateral lipid organization and domain shapes on a micrometer length scale including information of the lipid phase state. In the fluid-gel coexistence region of the pure binary lipid system, large-scale concentration fluctuations appear. Infrared spectral parameters were used to determine the peptide conformation adopted in the different lipid phases. The data show that the structure of the temperature-dependent lipid phases is significantly altered by the insertion of 2 to 5 mol% GD. At temperatures corresponding to the gel-fluid phase coexistence region the concentration fluctuations drastically decrease, and we observe domains in the giant unilamellar vesicles, which mainly disappear by the incorporation of 2 to 5 mol% GD. Further, the lipid matrix has the ability to modulate the conformation of the inserted polypeptide. The balance between double-helical and helical dimer structures of GD depends on the phospholipid chain length and phase state. A large hydrophobic mismatch, such as in gel phase one-component DSPC bilayers, leads to an increase in population of double-helical structures. Using an effective molecular sorting mechanism, a large hydrophobic mismatch can be avoided in the DMPC-DSPC lipid mixture, which leads to significant changes in the heterogeneous lipid structure and in polypeptide conformation.

Mobility of ions, sugar, and water in the cytoplasm of Xenopus oocytes expressing Na(+)-coupled sugar transporters (SGLT1)

A model was set up to study water transport in membrane proteins expressed in Xenopus oocytes. The model was tested experimentally using human and rabbit Na+-glucose cotransporters (SGLT1), and was used to explain controversies regarding unstirred layer effects. Cotransport of Na+, sugar and water was monitored by two-electrode voltage clamp and online measurements of oocyte volume. The specific resistance of the oocyte cytoplasm was found by means of microelectrodes to be 263 +/- 91 Omega cm (S.D., n = 52), or 2.5 times that of Kulori medium, in agreement with reported values of intracellular ion concentrations and diffusion constants. Osmotically induced volume and resistance changes were compatible with a model of the oocyte in which 37 +/- 17 % (S.D., n = 66) of the intracellular volume acts as a free solution while the remainder is inert, being occupied by organelles, etc. The model explains the results of several types of experiments: rapid changes in rates of water cotransport induced by changes in clamp voltage followed by osmotic equilibration in sugar-free conditions; volume changes induced by Na+ transport via the ionophore gramicidin; and uphill water transport. Ethanol (0.5 %) induced a marked swelling of the oocytes of about 16 pl x s(-1). If the specific inhibitor of SGLT1 phlorizin is added from stock solutions in ethanol, the effect of ethanol obfuscates the effects of the inhibitor. We conclude that the transport parameters derived for water cotransport by the SGLT1 can be attributed to the protein residing in the plasma membrane with no significant influences from unstirred layer effects.

Contribution of glycolytic and mitochondrial pathways in glucose-induced changes in islet respiration and insulin secretion

The different roles of glycolytic and mitochondrial pathways in glucose-induced metabolic activation and insulin secretion were studied in islets of Langerhans. Single islets were perifused with 3 mM glucose together with agents affecting the production or consumption of ATP. Subsequently, glucose was raised to 11 mM and the effects of the agents on metabolic and secretory responses were evaluated. Metabolism was monitored continuously with an oxygen-sensitive microelectrode inserted into the islet. Insulin secretion was determined by assaying insulin in perifusate with ELISA. Inhibitors of mitochondrial ATP production reduced the metabolic and secretory response to glucose. When glycolytic ATP production was reduced, initial but not sustained glucose-stimulated insulin release was observed. Inhibition of mitochondrial pyruvate transport reduced the glucose-induced decline in pO(2). Although mitochondrial metabolism was eventually similar to normal, insulin release was only 20% of normal. Increased energy expenditure also changed the kinetics of the glucose-induced decline in pO(2) and decreased the insulin release by 50%. In conclusion, glucose-induced enhancement of insulin release was only seen when the rise of the sugar concentration triggered a rapid and sustained increase of mitochondrial metabolism. This activation of mitochondrial metabolism required a good metabolic state prior to the glucose challenge.

Angiotensin II increases L-type Ca2+ current in gramicidin D-perforated adult rabbit ventricular myocytes: comparison with conventional patch-clamp method

The effects of angiotensin II (Ang II) on L-type Ca2+ current (I(Ca,L)) remains controversial. We studied the effects of Ang II on I(Ca,L) in single adult rabbit ventricular myocytes using a perforated patch-clamp technique with gramicidin D. Ang II increased I(Ca,L) in a concentration-dependent manner (EC(50)=0.75 nM). In contrast, in conventional whole-cell patch-calmp, I(Ca,L)ran down gradually and the I(Ca,L) response to Ang II was variable, suggesting the potential loss of diffusible components crucial for the Ang II-induced signaling process. An AT(1) antagonist, CV11974 (0.1 microM), completely inhibited the increase in I(Ca,L) induced by Ang II (0.1 microM), whereas an AT(2) antagonist, PD123319 (10 microM), did not influence the I(Ca,L) increase. Neither pre- nor after-treatment with a Na+/H+ exchange (NHE) inhibitor HOE642 (1 microM) affected the Ang II-induced increase in I(Ca,L). The protein kinase C (PKC) inhibitor chelerythrine (1 microM) did not affect the Ang II-induced I(Ca,L) increase. The present findings indicate that Ang II increases I(Ca,L) via AT(1) receptors in adult rabbit ventricular myocytes. Neither the activation of NHE nor PKC may contribute to the Ang II-induced activation of I(Ca,L).

Lipophilic cations: a group of model substrates for the multidrug-resistance transporter

The possibility that simple lipophilic cations such as tetraphenylphosphonium (TPA+), triphenylmethylphosphonium (TPMP+), and diphenyldimethylphosphonium (DDP+) are substrates for the multidrug-resistance transport protein, P-glycoprotein, was tested. Hamster cells transfected with and overexpressing mouse mdr1 or mouse mdr3 exhibit high levels of resistance to TPP+ and TPA+ (20-fold) and somewhat lower levels of resistance to TPMP+ and DDP+ (3-12-fold). Transfected cell clones expressing mdr1 or mdr3 mutants with decreased activity against drugs of the MDR spectrum (e.g., Vinca alkaloids and anthracyclines) also show reduced resistance to lipophilic cations. Studies with radiolabeled TPP+ and TPA+ demonstrate that increased resistance to cytotoxic concentrations of these lipophilic cations is correlated quantitatively with a decrease in intracellular accumulation in mdr1- and mdr3-transfected cells. This decreased intracellular accumulation is shown to be strictly dependent on intact intracellular nucleotide triphosphate pools and is reversed by verapamil, a known competitive inhibitor of P-glycoprotein. Taken together, these results demonstrate that lipophilic cations are a new class of substrates for P-glycoprotein and can be used to study its mechanism of action in homologous and heterologous systems.

Effects of the ionophores valinomycin, ionomycin and gramicidin A on the element compartmentation in cultured rat hepatocytes

The element compartmentation in cultured rat hepatocytes was studied by electron probe X-ray microanalysis of freeze-dried cryosections after exposure of the cells to the ionophores valinomycin, ionomycin or gramicidin A. The most striking effect of these ionophores is the decrease of the intracellular potassium/sodium ratio from values of approximately 10 under control conditions to values below 1 after application of the ionophores. Changes of sodium, potassium and chloride are similar in cytoplasm and nucleus. However, elemental changes are delayed or impeded in mitochondria with respect to the surrounding cytoplasm. The water portion of cytoplasm and mitochondria slightly increases. Besides that, each ionophore has specific effects on the intracellular ion distribution. As compared to gramicidin A and ionomycin, valinomycin does not change the intracellular chloride content. Ionomycin induces calcium accumulation in mitochondria. The cytotoxic effects of the studied ionophores on the intracellular element distribution are more complex than supposed from their ion selective properties in membranes.

On the origin of closing flickers in gramicidin channels: a new hypothesis

The submillisecond closing events (flickers) and the single channel conductances to protons (g(H)) were studied in native gramicidin A (gA) and in the SS and RR diastereoisomers of dioxolane-linked gA channels in planar bilayers. Bilayers were formed from glycerylmonooleate (GMO) in various solvents. In GMO/decane (thick) bilayers, the largest flicker frequency occurred in the SS channel (39 s(-1)), followed by the RR (4 s(-1)) and native gA channels (3 s(-1)). These frequencies were attenuated in GMO/squalene (thin) bilayers by 100-, 30-, and 70-fold in the SS, RR, and native gA channels, respectively. In thin bilayers, the average burst duration of native gA channels was 30-fold longer than in thick bilayers. The RR dioxolane-linked gA dimer "inactivated" in GMO/decane but not in squalene-containing bilayers. The mean closed time of flickers (approximately 0.12 ms) was essentially the same in various gA channels. In thin bilayers, g(H) values were larger by approximately 10% (SS), 30% (RR), and 20% (native gA) in relation to thick bilayers. It is concluded that flickers are not related to pre-dissociation or dissociation states of gA monomers, and do not seem to be caused by intrinsic conformational changes of channel proteins. It is proposed that flickers are caused by undulations of the bilayer that obliterate the openings of gA channels. Differences between flicker frequencies in various gA channels are likely to result from variations in channel geometries at the bilayer/channel interface. The smaller g(H) in thick bilayers suggests that the deformation of these bilayers around the gA channel creates a diffusional pathway next to the mouths of the channel that is longer and more restrictive than in thin GMO bilayers. A possible molecular interpretation for these effects is attempted.

Polyanions decelerate the kinetics of positively charged gramicidin channels as shown by sensitized photoinactivation

The effects of different anionic polymers on the kinetic properties of ionic channels formed by neutral gramicidin A (gA) and its positively charged analogs gramicidin-tris(2-aminoethyl)amine (gram-TAEA) and gramicidin-ethylenediamine (gram-EDA) in a bilayer lipid membrane were studied using a method of sensitized photoinactivation. The addition of Konig's polyanion caused substantial deceleration of the photoinactivation kinetics of gram-TAEA channels, which expose three positive charges to the aqueous phase at both sides of the membrane. In contrast, channels formed of gram-EDA, which exposes one positive charge, and neutral gA channels were insensitive to Konig's polyanion. The effect strongly depended on the nature of the polyanion added, namely: DNA, RNA, polyacrylic acid, and polyglutamic acid were inactive, whereas modified polyacrylic acid induced deceleration of the channel kinetics at high concentrations. In addition, DNA was able to prevent the action of Konig's polyanion. In single-channel experiments, the addition of Konig's polyanion resulted in the appearance of long-lived gram-TAEA channels. The deceleration of the gram-TAEA channel kinetics was ascribed to electrostatic interaction of the polyanion with gram-TAEA that reduces the mobility of gram-TAEA monomers and dimers in the membrane via clustering of channels.

Depolarization of the membrane potential by beta-lactams as a signal to induce autolysis

The effect of beta-lactam antibiotics that are known to inhibit cell wall biosynthesis and induce cell wall autolysis on the electrophysiological state of the plasma membrane in Streptomyces griseus was studied. Addition of various beta-lactam antibiotics induced a dose- and growth-stage-dependent depolarization of the membrane potential of Streptomyces griseus. The hydrolyzed biologically inactive derivative penicilloic acid had no depolarizing effect on the membrane potential. The ionophore gramicidin D, while depolarizing the membrane potential, also induced a dose-dependent increase in cell wall lysis. These observations suggest that alteration of the transmembrane potential could be an important signal in triggering cell wall autolysis of S. griseus.

Thallous ion movements through gramicidin channels incorporated in lipid monolayers supported by mercury

The potential independent limiting flux of hydrated Tl(+) ions through gramicidin (GR) channels incorporated in phospholipid monolayers self assembled on a hanging mercury-drop electrode is shown to be controlled both by diffusion and by a dehydration step. Conversely, the potential independent limiting flux of dehydrated Tl(+) ions stemming from Tl amalgam electro-oxidation is exclusively controlled by diffusion of thallium atoms within the amalgam. Modulating the charge on the polar heads of dioleoylphosphatidylserine (DOPS) by changing pH affects the limiting flux of hydrated Tl(+) ions to a notable extent, primarily by electrostatic interactions. The dipole potential of DOPS and dioleoylphosphatidylcholine (DOPC), positive toward the hydrocarbon tails, does not hinder the translocation step of Tl(+) ions to such an extent as to make it rate limiting. Consequently, incorporation in the lipid monolayer of phloretin, which decreases such a positive dipole potential, does not affect the kinetics of Tl(+) flux through GR channels. In contrast, the increase in the positive dipole potential produced by the incorporation of ketocholestanol causes the translocation step to contribute to the rate of the overall process. A model providing a quantitative interpretation of the kinetics of diffusion, dehydration-hydration, translocation, and charge transfer of the Tl(+)/Tl(0)(Hg) couple through GC channels incorporated in mercury-supported phospholipid monolayers is provided. A cut-off disk model yielding the profile of the local electrostatic potential created by an array of oriented dipoles located in the lipid monolayer along the axis of a cylindrical ion channel is developed.

Membrane dipole potential modulates proton conductance through gramicidin channel: movement of negative ionic defects inside the channel

The effect of membrane dipole potential on gramicidin channel activity in bilayer lipid membranes (BLMs) was studied. Remarkably, it appeared that proton conductance of gramicidin A (gA) channels responded to modulation of the dipole potential oppositely as compared with gA alkali metal cation conductance. In particular, the addition of phloretin, known to reduce the membrane dipole potential, resulted in a decrease in gA proton conductance, on one hand, and an increase in gA alkali metal conductance, on the other hand, whereas 6-ketocholestanol, the agent raising the membrane dipole potential, provoked an increase in gA proton conductance as opposed to a decrease in the alkali metal cation conductance. The peculiarity of the 6-ketocholestanol effect consisted in its dependence on the H(+) concentration. The experiments with the impermeant dipolar compound, phloridzin, showed that the response of proton transport through gramicidin channels to varying the membrane dipole potential did not change qualitatively if the dipole potential of only one monolayer or both monolayers of the BLM was altered. In contrast to gA proton conductance, the single-channel lifetime changed similarly with varying the membrane dipole potential, regardless of the kind of permeant cations (protons or potassium ions). The results of this study could be tentatively accounted for by an assumption that one of the rate-limiting steps of proton conduction through gramicidin channels represents, in fact, movement of negatively charged species (negative ionic defects) across a membrane.

The outward component of photoinduced current in chloroplasts of Peperomia metallica

The photoinduced currents in whole chloroplasts of Peperomia metallica were studied using suction electrodes and single-turnover flashes. The kinetic profile of the photocurrent contained a minor outward component (rise time, 100 micros). Local application (from the inside of the pipette) of a photosystem 2 inhibitor, DCMU, rapidly suppressed the outward current; conversely, addition of DCMU to the outer medium produced a transient stimulation of the outward component. Permeabilization of the tip-located membrane fragments with Triton X-100 eliminated the outward current, but had no significant influence on the inward current. The data suggest that the outward current originated in the tip-located nonruptured portions of the thylakoid membrane. Different involvement of two photosystems in the generation of the outward current indicates that granal thylakoids enriched with photosystem 2 are less susceptible to the rupture in the pipette tip as compared with stromal thylakoids.

Cell-type-specific GABA responses and chloride homeostasis in the cortex and amygdala

The GABA responses of fast-spiking (FS) interneurons and regular-spiking (RS) principal cells were studied using whole cell and perforated-patch recordings in slices of the basolateral amygdala, neo-, and perirhinal cortex. In these three areas, responses to exogenous and synaptically released GABA were abolished by GABA(A) receptor antagonists in FS cells but also included a GABA(B) component in RS cells. Moreover, E(GABA(A)) of FS and RS cells differed from the calculated E(Cl) (-61 mV), but in opposite direction (FS, -54 mV; RS, -72 mV). This was not due to a differential dialysis of FS and RS cells by the pipette solution because the discrepancy persisted when recordings were obtained with the perforated-patch-clamp technique, using the cation-selective ionophore gramicidin. Moreover, pharmacological inhibition of cation-chloride cotransporters revealed that the differing E(GABA(A)) of FS and RS neurons arises from cell-type-specific chloride homeostatic mechanisms. Indeed, the prevalent regulators of the intracellular chloride concentration are cotransporters that accumulate chloride in FS cells and extrude chloride in RS neurons. Thus, our results suggest that in the basolateral amygdala as well as in the parietal and perirhinal cortices, FS interneurons are more excitable than principal cells not only by virtue of their dissimilar electroresponsive properties but also because they express a different complement of GABA receptors and chloride homeostatic mechanisms.

Na,K-ATPase activity is required for formation of tight junctions, desmosomes, and induction of polarity in epithelial cells

Na,K-ATPase is a key enzyme that regulates a variety of transport functions in epithelial cells. In this study, we demonstrate a role for Na,K-ATPase in the formation of tight junctions, desmosomes, and epithelial polarity with the use of the calcium switch model in Madin-Darby canine kidney cells. Inhibition of Na,K-ATPase either by ouabain or potassium depletion prevented the formation of tight junctions and desmosomes and the cells remained nonpolarized. The formation of bundled stress fibers that appeared transiently in control cells was largely inhibited in ouabain-treated or potassium-depleted cells. Failure to form stress fibers correlated with a large reduction of RhoA GTPase activity in Na,K-ATPase-inhibited cells. In cells overexpressing wild-type RhoA GTPase, Na,K-ATPase inhibition did not affect the formation of stress fibers, tight junctions, or desmosomes, and epithelial polarity developed normally, suggesting that RhoA GTPase is an essential component downstream of Na,K-ATPase-mediated regulation of these junctions. The effects of Na,K-ATPase inhibition were mimicked by treatment with the sodium ionophore gramicidin and were correlated with the increased intracellular sodium levels. Furthermore, ouabain treatment under sodium-free condition did not affect the formation of junctions and epithelial polarity, suggesting that the intracellular Na(+) homeostasis plays a crucial role in generation of the polarized phenotype of epithelial cells. These results thus demonstrate that the Na,K-ATPase activity plays an important role in regulating both the structure and function of polarized epithelial cells.

Membrane-bound structure and alignment of the antimicrobial beta-sheet peptide gramicidin S derived from angular and distance constraints by solid state 19F-NMR

The antimicrobial properties of the cyclic beta-sheet peptide gramicidin S are attributed to its destabilizing effect on lipid membranes. Here we present the membrane-bound structure and alignment of a derivative of this peptide, based on angular and distance constraints. Solid-state 19F-NMR was used to study a 19F-labelled gramicidin S analogue in dimyristoylphosphatidylcholine bilayers at a lipid:peptide ratio of 80:1 and above. Two equivalent leucine side chains were replaced by the non-natural amino acid 4F-phenylglycine, which serves as a highly sensitive reporter on the structure and dynamics of the peptide backbone. Using a modified CPMG multipulse sequence, the distance between the two 19F-labels was measured from their homonuclear dipolar coupling as 6 A. in good agreement with the known backbone structure of natural gramicidin S in solution. By analyzing the anisotropic chemical shift of the 19F-labels in macroscopically oriented membrane samples, we determined the alignment of the peptide in the bilayer and described its temperature-dependent mobility. In the gel phase, the 19F-labelled gramicidin S is aligned symmetrically with respect to the membrane normal, i.e., with its cyclic beta-sheet backbone lying flat in the plane of the bilayer, which is fully consistent with its amphiphilic character. Upon raising the temperature to the liquid crystalline state, a considerable narrowing of the 19F-NMR chemical shift dispersion is observed, which is attributed the onset of global rotation of the peptide and further wobbling motions. This study demonstrates the potential of the 19F nucleus to describe suitably labelled polypeptides in membranes, requiring only little material and short NMR acquisition times.

GABA-mediated Ca2+ signalling in developing rat cerebellar Purkinje neurones

1. Cellular responses to GABA(A) receptor activation were studied in developing cerebellar Purkinje neurones (PNs) in brain slices obtained from 2- to 22-day-old rats. Two-photon fluorescence imaging of fura-2-loaded cells and perforated-patch recordings were used to monitor intracellular Ca2+ transients and to estimate the reversal potential of GABA-induced currents, respectively. 2. During the 1st postnatal week, focal application of GABA or the GABA(A) receptor agonist muscimol evoked transient increases in [Ca2+]i in immature PNs. These Ca2+ transients were reversibly abolished by the GABA(A) receptor antagonist bicuculline and by Ni2+, a blocker of voltage-activated Ca2+ channels. 3. Perforated-patch recordings were used to measure the reversal potential of GABA-evoked currents (E(GABA)) at different stages of development. It was found that E(GABA) was about -44 mV at postnatal day 3 (P3), it shifted to gradually more negative values during the 1st week and finally equilibrated at -87 mV at around the end of the 2nd postnatal week. This transition was well described by a sigmoidal function. The largest change in E(GABA) was -7 mV x day(-1), which occurred at around P6. 4. The transition in GABA-mediated signalling occurs during a period in which striking changes in PN morphology and synaptic connectivity are known to take place. Since such changes were shown to be Ca2+ dependent, we propose that GABA-evoked Ca2+ signalling is one of the critical determinants for the normal development of cerebellar PNs.

Structures of gramicidins A, B, and C incorporated into sodium dodecyl sulfate micelles

Gramicidins A, B, and C are the three most abundant, naturally occurring analogues of this family of channel-forming antibiotic. GB and GC differ from the parent pentadecapeptide, GA, by single residue mutations, W11F and W11Y, respectively. Although these mutations occur in the cation binding region of the channel, they do not affect monovalent cation specificity, but are known to alter cation-binding affinities, thermodynamic parameters of cation binding, conductance and the activation energy for ion transport. The structures of all three analogues incorporated into deuterated sodium dodecyl sulfate micelles have been obtained using solution state 2D-NMR spectroscopy and molecular modeling. For the first time, a rigorous comparison of the 3D structures of these analogues reveals that the amino acid substitutions do not have a significant effect on backbone conformation, thus eliminating channel differences as the cause of variations in transport properties. Variable positions of methyl groups in valine and leucine residues have been linked to molecular motions and are not likely to affect ion flow through the channel. Thus, it is concluded that changes in the magnitude and orientation of the dipole moment at residue 11 are responsible for altering monovalent cation transport.

Mode of antagonism of Brevibacillus brevis against Botrytis cinerea in vitro

AIMS

To assess the activity of Brevibacillus brevis (formerly Bacillus brevis) Nagano and the antibiotic it produces, gramicidin S, against the plant pathogen Botrytis cinerea.

METHODS AND RESULTS

Germination and growth of Bot. cinerea were assessed in the presence of B. brevis or gramicidin S in liquid media, on solid media and on leaf sections of Chinese cabbage. Germination was 10-fold more sensitive to gramicidin S than growth. Inhibition of Bot. cinerea was greater in liquid media compared with on solid media. Activity of gramicidin S against Bot. cinerea on leaf sections was much lower than in vitro. In vitro inhibition of Bot. cinerea by B. brevis Nagano was similar to equivalent levels of gramicidin.

CONCLUSIONS

Antibiosis, via gramicidin S, is the mode of antagonism exhibited by B. brevis Nagano against Bot. cinerea in vitro.

SIGNIFICANCE AND IMPACT OF THE STUDY

The mode of antagonism of B. brevis against Bot. cinerea was elucidated. The differing activity of gramicidin S against Bot. Cinerea in vitro and on leaf sections indicates one mechanism by which biocontrol activity may differ between laboratory and field conditions.

Dopamine-dependent synaptic plasticity in the striatal cholinergic interneurons

The striatum, the input stage of the basal ganglia, is a critical brain structure for the learning of stimulus-response habits as well as motor, perceptual, and cognitive skills. Roles of dopamine (DA) and acetylcholine (ACh) in this form of implicit memory have long been considered essential, but the underlying cellular mechanism is still unclear. By means of patch-clamp recordings from corticostriatal slices of the mouse, we studied whether the identified striatal cholinergic interneurons undergo long-term synaptic changes after tetanic stimulation of cortico- and thalamostriatal fibers. Electrical stimulation of the fibers revealed a depolarizing and hyperpolarizing postsynaptic potential in the striatal cholinergic interneurons. The early depolarizing phase was considered to be a cortico/thalamostriatal glutamatergic EPSP, and the hyperpolarizing component was considered to be an intrastriatally evoked GABAergic IPSP. Tetanic stimulation of cortico/thalamostriatal fibers was found to induce simultaneously occurring long-term potentiation (LTP) of the EPSPs as well as the disynaptically mediated IPSPs. The induction of LTP of EPSP required a rise in intracellular Ca(2+) concentration and dopamine D(5), but not D(2) receptor activation. Ca(2+)-permeable AMPA receptors might also play a part in the LTP induction. Blockade of NMDA receptors, metabotropic glutamate receptors, or serotonin receptors had no significant effects. The long-term enhancement of the disynaptic IPSPs was caused by a long-term increase in the occurrence rate but not the amplitude of disynaptically mediated IPSP in the striatal cholinergic interneurons. This dual mechanism of synaptic plasticity may be responsible for the long-term modulation of the cortico/thalamostriatal synaptic transmission.

'Chemical condoms' for the prevention of HIV infection: evaluation of novel agents against SHIV(89.6PD) in vitro and in vivo

BACKGROUND

Vaginal agents which are antiviral and/or inhibit the entry of HIV into the cell could prevent heterosexual transmission of HIV, and protect women who cannot negotiate condom use.

METHODS

Four agents have been investigated for activity in vitro and in vivo against SHIV(89.6PD): two anionic polymers, dextrin-2-sulphate (D2S) and PRO 2000 (P2K), and two virucidal agents; a non-ionic detergent, nonoxynol-9 (N9) and a cyclic peptide ionophore, gramicidin-D (GD). All four agents were investigated in rhesus macaques, using an intra-vaginal challenge of two inoculations of 1 x 104 50% tissue culture infectious doses (TCID)50 of SHIV(89.6PD).

RESULTS

D2S, P2K, GD and N9 all inhibited SHIV(89.6PD) in vitro. In vivo, three out of four control macaques were infected as judged by viral culture, seroconversion, DNA and RNA PCR; infection was confirmed in four out of eight macaques pre-treated with P2K, two out of four pre-treated with D2S, one out of four pre-treated with N9, two out of four pre-treated with GD and four out of four pre-treated with D2S + GD, a combination additive in vitro.

INTERPRETATION

D2S and PRO-2000, novel inhibitors of HIV entry, showed evidence of protection in vivo, comparable to that seen with the virucide, N9. These data, together with the results of phase I and phase II studies in healthy women which have shown minimal toxicity, support plans for a phase III efficacy trial of chemically simple inhibitors of HIV entry with low toxicity, for the prevention of HIV infection in women.

Extracellular ATP activates a P2 receptor in necturus erythrocytes during hypotonic swelling

We recently reported that ATP is released from Necturus erythrocytes via a conductive pathway during hypotonic swelling and that extracellular ATP potentiates regulatory volume decrease (RVD). This study was designed to determine whether extracellular ATP exerts its effect via a purinoceptor. This was accomplished using three different experimental approaches: 1) hemolysis studies to examine osmotic fragility, 2) a Coulter counter to assess RVD, and 3) the whole-cell patch-clamp technique to measure membrane currents. We found extracellular ATP and ATPgammaS, two P2 agonists, decreased osmotic fragility, enhanced cell volume recovery in response to hypotonic shock, and increased whole-cell currents. In addition, 2-methylthio-ATP potentiated RVD. In contrast, UTP, alpha,beta-methylene-ATP, and 2'-& 3'-O-(4-benzoyl-benzoyl) adenosine 5'-triphosphate and the P1 agonist adenosine had no effect regardless of experimental approach. Furthermore, the P2 antagonist suramin increased osmotic fragility, inhibited RVD, and reduced whole-cell conductance in swollen cells. Consistent with a previous study that indicated cell swelling activates a K+ conductance, suramin had no effect in the presence of gramicidin (a cationophore used to maintain a high K+ permeability). We also found the P2 antagonist pyridoxal-5-phosphate-6-azophenyl-2'4-disulfonic acid (PPADS) increased osmotic fragility; however, reactive blue 2 and the P1 antagonists caffeine and theophylline had no effect. Our results show that extracellular ATP activated a P2 receptor in Necturus erythrocytes during hypotonic swelling, which in turn potentiated RVD by stimulating K+ efflux. Pharmacological evidence suggested the presence of a P2X receptor subtype.

Role of quinine-sensitive ion channels in volume regulation in boar and bull spermatozoa

The ability to reverse swelling caused by hypo-osmotic stress is an important cell function; in spermatozoa, it is likely to be of consequence during ejaculation and also during the thawing process that terminates cryopreservation. In this study, the time course of boar and bull sperm volume changes after exposure to hypo-osmotic conditions at 39 degrees C was recorded. Cell volume measurements of washed sperm suspensions were performed electronically in Hepes-buffered saline solutions of 300 and 180 mosmol kg(-1) containing 2.5 mmol K(+) l(-1). Treatment with quinine in the presence or absence of the potassium ionophore valinomycin was used to determine whether potassium channels were involved in the reversal of swelling. After exposure to hypo-osmotic conditions, both bull and boar spermatozoa showed initial swelling (up to 200% and 140% of initial volume, respectively, within 5 min), which was subsequently partially reversed (to about 150% and 120%, respectively, after 20 min). Incubation with quinine led to an increase in swelling in both species. However, bull sperm volume was already maximal (up to 294%) after 30 s and declined thereafter, whereas boar sperm volume increased slowly to a maximum of about 220% after 20 min. Valinomycin treatment caused quinine-induced swelling in bull spermatozoa to decrease rapidly to control (no quinine, no valinomycin) values, whereas in quinine-treated boar spermatozoa it had an opposite, enhancing effect. Interpreting these results in the light of data from studies by others on a variety of cell types, it is proposed that swelling-activated potassium channels are involved in regulatory volume decrease in both species of spermatozoa, but that boar spermatozoa may contain fewer swelling-activated chloride channels than do bull spermatozoa.

Membrane potential changes after infection of monocytes by Toxoplasma gondii

Membrane potential changes in host cell plasma membrane were analyzed and the parasitophorous vacuole membrane (PVM) potential was characterized after infection by Toxoplasma gondii. Human monocytes infested by T. gondii were stained with two membrane potential sensitive dyes, DiOC(6)(3) carbocyanine and DiSBAC(2)(3) bis-oxonol, before fluorescence emission analysis by confocal laser scanning microscopy. After 24 and 48 h of infection, 34 and 39%, respectively, of monocytes showed several parasites (from two to six) per cell. At these infection times, significant decreases in cytoplasmic emissions were observed for both DiOC(6)(3) and DiSBAC(2)(3). Thus, hyperpolarisation of the host plasma membrane would occur consecutively to infection. Inside the parasitophorous vacuole, the fluorescence intensity of DiOC(6)(3) and DiSBAC(2)(3) increased significantly from 6 to 24 h after infection and the PVM became less polarised. Involvement of different ATPases in the membrane potential of infected monocytes was evaluated with ouabain, DCCD, omeprazole and sodium orthovanadate, ATPase inhibitors. All inhibitors induced a depolarisation of the plasma membrane. In the parasitophorous vacuole compartment, DCCD, omeprazole and sodium orthovanadate but not ouabain caused a significant depolarisation of the PVM, suggesting that H(+), H(+)/K(+) and P-type ATPases were at the origin of the PVM potential. This is the first report showing the presence of ion transporters in the T. gondii PVM and the existence of at least two members of the P-type family of ion pumps: an electrogenic H(+)ATPase and an electroneutral H(+)/K(+) ATPase.

Diversity in antistaphylococcal mechanisms among membrane-targeting antimicrobial peptides

Many antimicrobial peptides permeabilize the bacterial cytoplasmic membrane. However, it is unclear how membrane permeabilization and antimicrobial activity are related for distinct peptides. This study investigated the relationship between Staphylococcus aureus membrane permeabilization and cell death due to the following antistaphylococcal peptides: thrombin-induced platelet microbicidal protein 1 (tPMP-1), gramicidin D, and protamine. Isogenic S. aureus strains ISP479C and ISP479R (tPMP-1 susceptible and resistant, respectively), were loaded with the fluorochrome calcein and exposed to a range of concentrations of each peptide. Flow cytometry was then used to monitor membrane permeabilization by quantifying the release of preloaded calcein. Killing was determined by quantitative culture at time points simultaneous to measurement of membrane permeabilization. Membrane permeabilization and killing caused by tPMP-1 occurred in a time- and concentration-dependent manner, reflecting the intrinsic tPMP-1 susceptibilities of ISP479C and ISP479R. In comparison, gramicidin D killed both S. aureus strains to equivalent extents in a concentration-dependent manner between 0.5 to 50 microg/ml, but cell permeabilization only occurred at the higher peptide concentrations (25 and 50 microg/ml). Protamine permeabilized, but did not kill, either strain at concentrations up to 10 mg/ml. Regression analyses revealed different relationships between membrane permeabilization and staphylocidal activity for the distinct antimicrobial peptides. Taken together, these findings demonstrate that permeabilization, per se, does not invariably result in staphylococcal death due to distinct antimicrobial peptides. Thus, although each of these peptides interacts with the S. aureus cytoplasmic membrane, diversity exists in their mechanisms of action with respect to the relationship between membrane permeabilization and staphylocidal activity.

Cytolytic peptides induce biphasic permeability changes in mammalian cell membranes

The cytolytic peptides melittin and gramicidin S are naturally occurring agents that provide a comparative model for studies of complement, immunotoxin and cell-mediated membrane permeability. Most attempts to characterize cytolytic peptides have used model membrane systems including phospholipid vesicles or erythrocytes. Membrane vesicles permit the use of self-quenching concentrations of fluorescent permeability markers, while erythrocytes release measurable hemoglobin. Attempts at measuring early membrane permeability changes in nucleated mammalian cells have been limited. To measure the kinetics of mammalian cell membrane permeability changes induced by cytolytic peptides, we developed a 96-well fluorescence cytolysis assay using the cytoplasmic fluorescent dye calcein as the membrane permeability marker. To facilitate rapid assessment of membrane permeability, trypan blue was added to the assay solution to quench (a) released fluorescence and (b) retained intracellular fluorescence. Trypan blue also provided a complementary visual assessment of cell viability. Using this assay, a detailed kinetic analysis demonstrated permeability of the cell membranes within seconds of exposure to the cytolytic peptides. The rapid permeabilization of the cell membranes was confirmed by flow cytometry using the calcium indicator dye fluo-3. The assay also demonstrated a second slower phase of marker release over the next several hours. The fluorescence cytolysis assay was able to reliably detect the biphasic permeability changes associated with the melittin and gramicidin S peptides suggesting the potential utility of this assay in the assessment of other cytolytic agents.

Post-Episode Depression of GABAergic Transmission in Spinal Neurons of the Chick Embryo

Whole cell recordings were obtained from ventral horn neurons in spontaneously active spinal cords isolated from the chick embryo [ embryonic days 10 to 11 ( E10–E11)] to examine the post-episode depression of GABAergic transmission. Spontaneous activity occurred as recurrent, rhythmic episodes approximately 60 s in duration with 10- to 15-min quiescent inter-episode intervals. Current-clamp recording revealed that episodes were followed by a transient hyperpolarization (7 ± 1.2 mV, mean ± SE), which dissipated as a slow (0.5–1 mV/min) depolarization until the next episode. Local application of bicuculline 8 min after an episode hyperpolarized spinal neurons by 6 ± 0.8 mV and increased their input resistance by 13%, suggesting the involvement of GABAergic transmission. Gramicidin perforated-patch recordings showed that the GABAa reversal potential was above rest potential ( E GABAa = −29 ± 3 mV) and allowed estimation of the physiological intracellular [Cl−] = 50 mM. In whole cell configuration (with physiological electrode [Cl−]), two distinct types of endogenous GABAergic currents ( I GABAa) were found during the inter-episode interval. The first comprised TTX-resistant, asynchronous miniature postsynaptic currents (mPSCs), an indicator of quantal GABA release (up to 42% of total mPSCs). The second (tonic I GABAa) was complimentary to the slow membrane depolarization and may arise from persistent activation of extrasynaptic GABAa receptors. We estimate that approximately 10 postsynaptic channels are activated by a single quantum of GABA release during an mPSC and that about 30 extrasynaptic GABAa channels are required for generation of the tonic I GABAa in ventral horn neurons. We investigated the post-episode depression of I GABAa by local application of GABA or isoguvacine (100 μM, for 10–30 s) applied before and after an episode at holding potentials ( V hold) −60 mV. The amplitude of the evoked I GABA was compared after clamping the cell during the episode at one of three different V hold: −60 mV, below E GABAa resulting in Cl− efflux; −30 mV, close to E GABAa with minimal Cl− flux; and 0 mV, above E GABAa resulting in Cl− influx during the episode. The amplitude of the evoked I GABA changed according to the direction of Cl− flux during the episode: at −60 mV a 41% decrease, at −30 mV a 4% reduction, and at 0 mV a 19% increase. These post-episode changes were accompanied by shifts of E GABAa of −10, −1.2, and +7 mV, respectively. We conclude that redistribution of intracellular [Cl−] during spontaneous episodes is likely to be an important postsynaptic mechanism involved in the post-episode depression of GABAergic transmission in chick embryo spinal neurons.

Covalently linked gramicidin channels: effects of linker hydrophobicity and alkaline metals on different stereoisomers

The direct role of the dioxolane group on the gating and single-channel conductance of different stereoisomers of the dioxolane-linked gramicidin A (gA) channels reconstituted in planar lipid bilayers was investigated. Four different covalently linked gA dimers were synthesized. In two of them, the linker was the conventional dioxolane described previously (SS and RR channels). Two gAs were covalently linked with a novel modified dioxolane group containing a retinal attachment (ret-SS and ret-RR gA dimers). These proteins also formed ion channels in lipid bilayers and were selective for monovalent cations. The presence of the bulky and hydrophobic retinal group immobilizes the dioxolane linker in the bilayer core preventing its rotation into the hydrophilic lumen of the pore. In 1 M HCl the gating kinetics of the SS or RR dimers were indistinguishable from their retinal counterparts; the dwell-time distributions of the open and closed states in the SS and ret-SS were basically the same. In particular, the inactivation of the RR was not prevented by the presence of the retinal group. It is concluded that neither the fast closing events in the SS or RR dimers nor the inactivation of the RR are likely to be a functional consequence of the flipping of the dioxolane inside the pore of the channel. On the other hand, the inactivation of the RR dimer was entirely eliminated when alkaline metals (Cs(+) or K(+)) were the permeating cations in the channel. In fact, the open state of the RR channel became extremely stable, and the gating characteristics of both the SS and RR channels were different from what was seen before with permeating protons. As in HCl, the presence of a retinal in the dioxolane linker did not affect the gating behavior of the SS and RR in Cs(+)- or K(+)-containing solutions. Alternative hypotheses concerning the gating of linked gA dimers are discussed.