Intrinsic Lipid Curvature and Bilayer Elasticity as Regulators of Channel Function: A Comparative Single-Molecule Study

Perturbations in bilayer material properties (thickness, lipid intrinsic curvature and elastic moduli) modulate the free energy difference between different membrane protein conformations, thereby leading to changes in the conformational preferences of bilayer-spanning proteins. To further explore the relative importance of curvature and elasticity in determining the changes in bilayer properties that underlie the modulation of channel function, we investigated how the micelle-forming amphiphiles Triton X-100, reduced Triton X-100 and the HII lipid phase promoter capsaicin modulate the function of alamethicin and gramicidin channels. Whether the amphiphile-induced changes in intrinsic curvature were negative or positive, amphiphile addition increased gramicidin channel appearance rates and lifetimes and stabilized the higher conductance states in alamethicin channels. When the intrinsic curvature was modulated by altering phospholipid head group interactions, however, maneuvers that promote a negative-going curvature stabilized the higher conductance states in alamethicin channels but destabilized gramicidin channels. Using gramicidin channels of different lengths to probe for changes in bilayer elasticity, we found that amphiphile adsorption increases bilayer elasticity, whereas altering head group interactions does not. We draw the following conclusions: first, confirming previous studies, both alamethicin and gramicidin channels are modulated by changes in lipid bilayer material properties, the changes occurring in parallel yet differing dependent on the property that is being changed; second, isolated, negative-going changes in curvature stabilize the higher current levels in alamethicin channels and destabilize gramicidin channels; third, increases in bilayer elasticity stabilize the higher current levels in alamethicin channels and stabilize gramicidin channels; and fourth, the energetic consequences of changes in elasticity tend to dominate over changes in curvature.

Gramicidin, a Bactericidal Antibiotic, Is an Antiproliferative Agent for Ovarian Cancer Cells

Background and Objectives: Gramicidin, a bactericidal antibiotic used in dermatology and ophthalmology, has recently garnered attention for its inhibitory actions against cancer cell growth. However, the effects of gramicidin on ovarian cancer cells and the underlying mechanisms are still poorly understood. We aimed to elucidate the anticancer efficacy of gramicidin against ovarian cancer cells. Materials and Methods: The anticancer effect of gramicidin was investigated through an in vitro experiment. We analyzed cell proliferation, DNA fragmentation, cell cycle arrest and apoptosis in ovarian cancer cells using WST-1 assay, terminal deoxynucleotidyl transferase dUTP nick and labeling (TUNEL), DNA agarose gel electrophoresis, flow cytometry and western blot. Results: Gramicidin treatment induces dose- and time-dependent decreases in OVCAR8, SKOV3, and A2780 ovarian cancer cell proliferation. TUNEL assay and DNA agarose gel electrophoresis showed that gramicidin caused DNA fragmentation in ovarian cancer cells. Flow cytometry demonstrated that gramicidin induced cell cycle arrest. Furthermore, we confirmed via Western blot that gramicidin triggered apoptosis in ovarian cancer cells. Conclusions: Our results strongly suggest that gramicidin exerts its inhibitory effect on cancer cell growth by triggering apoptosis. Conclusively, this study provides new insights into the previously unexplored anticancer properties of gramicidin against ovarian cancer cells.

β-Turn editing in Gramicidin S: Activity impact on replacing proline α-carbon with stereodynamic nitrogen

Gramicidin S, natural antimicrobial peptide is used commercially in medicinal lozenges for sore throat and Gram-negative and Gram-positive bacterial infections. However, its clinical potential is limited to topical applications because of its high red blood cells (RBC) cytotoxicity. Given the importance of developing potential antibiotics and inspired by the cyclic structure and druggable features of Gramicidin S, we edited proline α-carbon with stereodynamic nitrogen to examine the direct impact on biological activity and cytotoxicity with respect to prolyl counterpart. Natural Gramicidin S (12), proline-edited peptides 13-16 and wild-type d-Phe-d-Pro β-turn mimetics (17 and 18) were synthesized using solid phase peptide synthesis and investigated their activity against clinically relevant bacterial pathogens. Interestingly, mono-proline edited analogous peptide 13 showed moderate improvement in antimicrobial activity against E. coli ATCC 25922 and K.pneumoniae BAA 1705 as compared to Gramicidin S. Furthermore, proline edited peptide 13 exhibited equipotent antimicrobial effect against MDR S. aureus and Enterococcus spp. Analysis of cytotoxicity against VERO cells and RBC, reveals that proline edited peptides showed two-fivefold lesser cytotoxicity than the counterpart Gramicidin S. Our study suggests that introducing single azPro/Pro mutation in Gramicidin S marginally improved the activity and lessens the cytotoxicity as compared with the parent peptide.

Screening for bilayer-active and likely cytotoxic molecules reveals bilayer-mediated regulation of cell function

A perennial problem encountered when using small molecules (drugs) to manipulate cell or protein function is to assess whether observed changes in function result from specific interactions with a desired target or from less specific off-target mechanisms. This is important in laboratory research as well as in drug development, where the goal is to identify molecules that are unlikely to be successful therapeutics early in the process, thereby avoiding costly mistakes. We pursued this challenge from the perspective that many bioactive molecules (drugs) are amphiphiles that alter lipid bilayer elastic properties, which may cause indiscriminate changes in membrane protein (and cell) function and, in turn, cytotoxicity. Such drug-induced changes in bilayer properties can be quantified as changes in the monomer↔dimer equilibrium for bilayer-spanning gramicidin channels. Using this approach, we tested whether molecules in the Pathogen Box (a library of 400 drugs and drug-like molecules with confirmed activity against tropical diseases released by Medicines for Malaria Venture to encourage the development of therapies for neglected tropical diseases) are bilayer modifiers. 32% of the molecules in the Pathogen Box were bilayer modifiers, defined as molecules that at 10 µM shifted the monomer↔dimer equilibrium toward the conducting dimers by at least 50%. Correlation analysis of the molecules' reported HepG2 cell cytotoxicity to bilayer-modifying potency, quantified as the shift in the gramicidin monomer↔dimer equilibrium, revealed that molecules producing <25% change in the equilibrium had significantly lower probability of being cytotoxic than molecules producing >50% change. Neither cytotoxicity nor bilayer-modifying potency (quantified as the shift in the gramicidin monomer↔dimer equilibrium) was well predicted by conventional physico-chemical descriptors (hydrophobicity, polar surface area, etc.). We conclude that drug-induced changes in lipid bilayer properties are robust predictors of the likelihood of membrane-mediated off-target effects, including cytotoxicity.

Effects of Calcium Ions on the Antimicrobial Activity of Gramicidin A

Gramicidin A (gA) is a linear antimicrobial peptide that can form a channel and specifically conduct monovalent cations such as H⁺ across the lipid membrane. The antimicrobial activity of gA is associated with the formation of hydroxyl free radicals and the imbalance of NADH metabolism, possibly a consequence caused by the conductance of cations. The ion conductivity of gramicidin A can be blocked by Ca²⁺ ions. However, the effect of Ca²⁺ ions on the antimicrobial activity of gA is unclear. To unveil the role of Ca²⁺ ions, we examined the effect of Ca²⁺ ions on the antimicrobial activity of gramicidin A against Staphylococcus aureus (S. aureus). Results showed that the antimicrobial mechanism of gA and antimicrobial activity by Ca²⁺ ions are concentration-dependent. At the low gA concentration (≤1 μM), the antimicrobial mechanism of gA is mainly associated with the hydroxyl free radical formation and NADH metabolic imbalance. Under this mode, Ca²⁺ ions can significantly inhibit the hydroxyl free radical formation and NADH metabolic imbalance. On the other hand, at high gA concentration (≥5 μM), gramicidin A acts more likely as a detergent. Gramicidin A not only causes an increase in hydroxyl free radical levels and NAD⁺/NADH ratios but also induces the destruction of the lipid membrane composition. At this condition, Ca²⁺ ions can no longer reduce the gA antimicrobial activity but rather enhance the bacterial killing ability of gramicidin A.

Cannabidiol increases gramicidin current in human embryonic kidney cells: An observational study

Gramicidin is a monomeric protein that is thought to non-selectively conduct cationic currents and water. Linear gramicidin is considered an antibiotic. This function is considered to be mediated by the formation of pores within the lipid membrane, thereby killing bacterial cells. The main non-psychoactive active constituent of the cannabis plant, cannabidiol (CBD), has recently gained interest, and is proposed to possess various potential therapeutic properties, including being an antibiotic. We previously determined that CBD’s activity on ion channels could be, in part, mediated by altering membrane biophysical properties, including elasticity. In this study, our goal was to determine the empirical effects of CBD on gramicidin currents in human embryonic kidney (HEK) cells, seeking to infer potential direct compound-protein interactions. Our results indicate that gramicidin, when applied to the extracellular HEK cell membrane, followed by CBD perfusion, increases the gramicidin current.

Synthesis and biological evaluation of backbone-aminated analogues of gramicidin S

We report the parallel synthesis of gramicidin S derivatives featuring backbone N-amino substituents. Analogues were prepared by incorporation of N-amino dipeptide subunits on solid support. Nine backbone-aminated macrocycles were evaluated for growth inhibitory activity against ESKAPE pathogens and hemolytic activity against human red blood cells. Diamination of the Orn residues in the β-strand region of gramicidin S was found to enhance broad-spectrum antimicrobial activity without a corresponding increase in hemolytic activity.

Characterization of the transient fluorescence wave phenomenon that occurs during H2 production in Chlamydomonas reinhardtii

The redox state of the plastoquinone (PQ) pool in sulfur-deprived, H2-producing Chlamydomonas reinhardtii cells was studied using single flash-induced variable fluorescence decay kinetics. During H2 production, the fluorescence decay kinetics exhibited an unusual post-illumination rise of variable fluorescence, giving a wave-like appearance. The wave showed the transient fluorescence minimum at ~60 ms after the flash, followed by a rise, reaching the transient fluorescence maximum at ~1 s after the flash, before decaying back to the initial fluorescence level. Similar wave-like fluorescence decay kinetics have been reported previously in anaerobically incubated cyanobacteria but not in green algae. From several different electron and proton transfer inhibitors used, polymyxin B, an inhibitor of type II NAD(P)H dehydrogenase (NDA2), had the effect of eliminating the fluorescence wave feature, indicating involvement of NDA2 in this phenomenon. This was further confirmed by the absence of the fluorescence wave in the Δnda2 mutant lacking NDA2. Additionally, Δnda2 mutants have also shown delayed and diminished H2 production (only 23% if compared with the wild type). Our results show that the fluorescence wave phenomenon in C. reinhardtii is observed under highly reducing conditions and is induced by the NDA2-mediated electron flow from the reduced stromal components to the PQ pool. Therefore, the fluorescence wave phenomenon is a sensitive probe for the complex network of redox reactions at the PQ pool level in the thylakoid membrane. It could be used in further characterization and improvement of the electron transfer pathways leading to H2 production in C. reinhardtii.

The effect of gramicidin inclusions on the local order of membrane components

We study the local effect of the antimicrobial peptide Gramicidin A on bilayers composed of lipids or surfactants using nuclear magnetic resonance spectroscopy and wide-angle X-ray scattering, techniques that probe the orientational and positional order of the alkyl chains, respectively. The two types of order vary with temperature and peptide concentration in complex ways which depend on the membrane composition, highlighting the subtlety of the interaction between inclusions and the host bilayer. The amplitude of the variation is relatively low, indicating that the macroscopic constants used to describe the elasticity of the bilayer are unlikely to change with the addition of peptide.

Mechanistic differences in the uptake of salicylic acid glucose conjugates by vacuolar membrane-enriched vesicles isolated from Arabidopsis thaliana

Salicylic acid (SA) is a plant hormone involved in a number of physiological responses including both local and systemic resistance of plants to pathogens. In Arabidopsis, SA is glucosylated to form either SA 2-O-β-d-glucose (SAG) or SA glucose ester (SGE). In this study, we show that SAG accumulates in the vacuole of Arabidopsis, while the majority of SGE was located outside the vacuole. The uptake of SAG by vacuolar membrane-enriched vesicles isolated from Arabidopsis was stimulated by the addition of MgATP and was inhibited by both vanadate (ABC transporter inhibitor) and bafilomycin A₁ (vacuolar H⁺ -ATPase inhibitor), suggesting that SAG uptake involves both an ABC transporter and H⁺ -antiporter. Despite its absence in the vacuole, we observed the MgATP-dependent uptake of SGE by Arabidopsis vacuolar membrane-enriched vesicles. SGE uptake was not inhibited by vanadate but was inhibited by bafilomycin A₁ and gramicidin D providing evidence that uptake was dependent on an H⁺ -antiporter. The uptake of both SAG and SGE was also inhibited by quercetin and verapamil (two known inhibitors of multidrug efflux pumps) and salicin and arbutin. MgATP-dependent SAG and SGE uptake exhibited Michaelis-Menten-type saturation kinetics. The vacuolar enriched-membrane vesicles had a 46-fold greater affinity and a 10-fold greater transport activity with SGE than with SAG. We propose that in Arabidopsis, SAG is transported into the vacuole to serve as a long-term storage form of SA while SGE, although also transported into the vacuole, is easily hydrolyzed to release the active hormone which can then be remobilized to other cellular locations.

Surfactant Releases Internal Calcium Stores in Neutrophils by G Protein–Activated Pathway

Pulmonary surfactant with surfactant-associated proteins (PS+SAP) decreases pulmonary inflammation by suppressing neutrophil activation. We have observed that PS+SAP inserts channels into artificial membranes, depolarizes neutrophils, and depresses calcium influx and function in stimulated neutrophils. We hypothesize that PS+SAP suppresses neutrophil activation by depletion of internal Ca++ stores and that PS+SAP induces depletion through release of Ca++ stores and through inhibition of Ca++ influx. Our model predicts that PS+SAP releases Ca++ stores through insertion of channels, depolarization of neutrophils, and activation of a G protein–dependent pathway. If the model of channel insertion and membrane depolarization is accurate, then gramicidin—a channel protein with properties similar to those of PS+SAP—is expected to mimic these effects. Human neutrophils were monitored for [Ca++] responses after exposure to one of two different PS+SAP preparations, a PS-SAP preparation, gramicidin alone, and gramicidin reconstituted with phospholipid (PLG). [Ca++] responses were reexamined following preexposure to inhibitors of internal Ca++ release or the G protein pathway. We observed that (i) 1% PS+SAP—but not PS-SAP—causes transient increase of neutrophil [Ca++] within seconds of exposure; (ii) 1% PLG—but not gramicidin alone—closely mimics the effect of PS+SAP on Ca++ response; (iii) PS+SAP and PLG equally depolarize neutrophils; (iv) direct inhibition of internal Ca++ stores releases or of G protein activation suppresses Ca++ responses to PS+SAP and PLG; and (v) preexposure to either PS+SAP or PLG inhibits Ca++ influx following fMLP stimulation. We conclude that PS+SAP independently depolarizes neutrophils, releases Ca++ from internal stores by a G protein-mediated pathway, and alters subsequent neutrophil response to physiologic stimulants by depleting internal Ca++ stores and by inhibiting Ca++ influx during subsequent fMLP activation. The mimicking of these results by PLG supports the hypothesis that PS+SAP initiates depolarization via channel insertion into neutrophil plasma membrane.

Assessment of anoxia tolerance and photoperiod dependence of GABAergic polarity in the pond snail Lymnaea stagnalis

The pond snail Lymnaea stagnalis is reported to be anoxia-tolerant and if the tolerance mechanism is similar to that of the anoxia-tolerant painted turtle, GABA should play an important role. A potentially confounding factor investigating the role of GABA in anoxia tolerance are reports that GABA has both inhibitory and excitatory effects within L. stagnalis central ganglion. We therefore set out to determine if seasonality or photoperiod has an impact on: 1) the anoxia-tolerance of the intact pond snail, and 2) the response of isolated neuroganglia cluster F neurons to exogenous GABA application. L. stagnalis maintained on a natural summer light cycle were unable to survive any period of anoxic exposure, while those maintained on a natural winter light cycle survived a maximum of 4h. Using intracellular sharp electrode recordings from pedal ganglia cluster F neurons we show that there is a photoperiod dependent shift in the response to GABA. Snails exposed to a 16h:8h light:dark cycle in an environmental chamber (induced summer phenotype) exhibited hyperpolarizing inhibitory responses and those exposed to a 8h:16h light:dark cycle (induced winter phenotype) exhibited depolarizing excitatory responses to GABA application. Using gramicidin-perforated patch recordings we also found a photoperiod dependent shift in the reversal potential for GABA. We conclude that the opposing responses of L. stagnalis central neurons to GABA results from a shift in intracellular chloride concentration that is photoperiod dependent and is likely mediated through the relative efficacy of cation chloride co-transporters. Although the physiological ramifications of the photoperiod dependent shift are unknown this work potentially has important implications for the impact of artificial light pollution on animal health.

pH-Insensitive FRET Voltage Dyes

Many high-throughput ion channel assays require the use of voltage-sensitive dyes to detect channel activity in the presence of test compounds. Dye systems employing Förster resonance energy transfer (FRET) between 2 membrane-bound dyes are advantageous in combining high sensitivity, relatively fast response, and ratiometric output. The most widely used FRET voltage dye system employs a coumarin fluorescence donor whose excitation spectrum is pH dependent. The authors have validated a new class of voltage-sensitive FRET donors based on a pyrene moiety. These dyes are significantly brighter than CC2-DMPE and are not pH sensitive in the physiological range. With the new dye system, the authors demonstrate a new high-throughput assay for the acid-sensing ion channel (ASIC) family. They also introduce a novel method for absolute calibration of voltage-sensitive dyes, simultaneously determining the resting membrane potential of a cell. ( Journal of Biomolecular Screening 2007:656-667)

Connectivity of the intracytoplasmic membrane of Rhodobacter sphaeroides: a functional approach

The photosynthetic apparatus in the bacterium Rhodobacter sphaeroides is mostly present in intracytoplasmic membrane invaginations. It has long been debated whether these invaginations remain in topological continuity with the cytoplasmic membrane, or form isolated chromatophore vesicles. This issue is revisited here by functional approaches. The ionophore gramicidin was used as a probe of the relative size of the electro-osmotic units in isolated chromatophores, spheroplasts, or intact cells. The decay of the membrane potential was monitored from the electrochromic shift of carotenoids. The half-time of the decay induced by a single channel in intact cells was about 6 ms, thus three orders of magnitude slower than in isolated chromatophores. In spheroplasts obtained by lysis of the cell wall, the single channel decay was still slower (~23 ms) and the sensitivity toward the gramicidin concentration was enhanced 1,000-fold with respect to isolated chromatophores. These results indicate that the area of the functional membrane in cells or spheroplasts is about three orders of magnitude larger than that of isolated chromatophores. Intracytoplasmic vesicles, if present, could contribute to at most 10% of the photosynthetic apparatus in intact cells of Rba. sphaeroides. Similar conclusions were obtained from the effect of a ∆pH-induced diffusion potential in intact cells. This caused a large electrochromic response of carotenoids, of similar amplitude as the light-induced change, indicating that most of the system is sensitive to a pH change of the external medium. A single internal membrane and periplasmic space may offer significant advantages concerning renewal of the photosynthetic apparatus and reallocation of the components shared with other bioenergetic pathways.

The OpenPicoAmp: an open-source planar lipid bilayer amplifier for hands-on learning of neuroscience

Understanding the electrical biophysical properties of the cell membrane can be difficult for neuroscience students as it relies solely on lectures of theoretical models without practical hands on experiments. To address this issue, we developed an open-source lipid bilayer amplifier, the OpenPicoAmp, which is appropriate for use in introductory courses in biophysics or neurosciences at the undergraduate level, dealing with the electrical properties of the cell membrane. The amplifier is designed using the common lithographic printed circuit board fabrication process and off-the-shelf electronic components. In addition, we propose a specific design for experimental chambers allowing the insertion of a commercially available polytetrafluoroethylene film. We provide a complete documentation allowing to build the amplifier and the experimental chamber. The students hand-out giving step-by step instructions to perform a recording is also included. Our experimental setup can be used in basic experiments in which students monitor the bilayer formation by capacitance measurement and record unitary currents produced by ionic channels like gramicidin A dimers. Used in combination with a low-cost data acquisition board this system provides a complete solution for hands-on lessons, therefore improving the effectiveness in teaching basic neurosciences or biophysics.

Gramicidin D enhances the antibacterial activity of fluoride

Fluoride is a toxic anion found in many natural environments. One of the major bacterial defenses against fluoride is the cell envelope, which limits passage of the membrane-impermeant fluoride anion. Accordingly, compounds that enhance the permeability of bacterial membranes to fluoride should also enhance fluoride toxicity. In this study, we demonstrate that the pore-forming antibiotic gramicidin D increases fluoride uptake in Bacillus subtilis and that the antibacterial activity of this compound is potentiated by fluoride. Polymyxin B, another membrane-targeting antibiotic with a different mechanism of action, shows no such improvement. These results, along with previous findings, indicate that certain compounds that destabilize bacterial cell envelopes can enhance the toxicity of fluoride.

Regulation of action potential waveforms by axonal GABAA receptors in cortical pyramidal neurons

GABAA receptors distributed in somatodendritic compartments play critical roles in regulating neuronal activities, including spike timing and firing pattern; however, the properties and functions of GABAA receptors at the axon are still poorly understood. By recording from the cut end (bleb) of the main axon trunk of layer -5 pyramidal neurons in prefrontal cortical slices, we found that currents evoked by GABA iontophoresis could be blocked by picrotoxin, indicating the expression of GABAA receptors in axons. Stationary noise analysis revealed that single-channel properties of axonal GABAA receptors were similar to those of somatic receptors. Perforated patch recording with gramicidin revealed that the reversal potential of the GABA response was more negative than the resting membrane potential at the axon trunk, suggesting that GABA may hyperpolarize the axonal membrane potential. Further experiments demonstrated that the activation of axonal GABAA receptors regulated the amplitude and duration of action potentials (APs) and decreased the AP-induced Ca2+ transients at the axon. Together, our results indicate that the waveform of axonal APs and the downstream Ca2+ signals are modulated by axonal GABAA receptors.

Effects of low-intensity ultrasound on gramicidin D-induced erythrocyte edema

OBJECTIVES

To determine whether low-intensity ultrasound (US) can reduce red blood cell (RBC) edema and, if so, whether the US activity is associated with aquaporin 1 (AQP-1), a water channel in the cell membrane.

METHODS

Red blood cell edema was induced by gramicidin D treatment at 40 ng/mL for 20 minutes and evaluated by a hematocrit assay. Low-intensity continuous wave US at 1 MHz was applied to RBCs for the last 10 minutes of gramicidin D treatment. To determine whether US activity was associated with AQP-1, RBCs were treated with 40 μM mercuric chloride (HgCl(2)), an AQP-1 inhibitor, for 20 minutes at the time of gramicidin D treatment. Posttreatment morphologic changes in RBCs were observed by actin staining with phalloidin.

RESULTS

Red blood cell edema increased significantly with gramicidin D at 20 (1.8%), 40 (6.7%), 60 (16.7%), and 80 (11.3%) ng/mL, reaching a peak at 60 ng/mL, compared to the control group (20 ng/mL, P = .019; 40, 60, and 80 ng/mL, P < .001). No significant RBC hemolysis was observed in any group. Edema induced by gramicidin D at 40 ng/mL was significantly reduced by US at 30 (3.4%; P = .003), 70 (4.4%; P = .001), and 100 (2.9%; P = .001) mW/cm(2). Subsequent experiments showed that edema reduction by US ranged from 7% to 10%. Cotreatment with HgCl(2) partially reversed the US effect and showed a significantly different level of edema compared to gramicidin D-alone and US-cotreated groups (P = .001). These results were confirmed by microscopic observation of RBC morphologic changes.

CONCLUSIONS

Low-intensity US could reduce gramicidin D-induced RBC edema, and its effect appeared to at least partly involve regulation of AQP-1 activity. These results suggest that low-intensity US can be used as an alternative treatment to control edema and related disorders.

(19)F NMR screening of unrelated antimicrobial peptides shows that membrane interactions are largely governed by lipids

Many amphiphilic antimicrobial peptides permeabilize bacterial membranes via successive steps of binding, re-alignment and/or oligomerization. Here, we have systematically compared the lipid interactions of two structurally unrelated peptides: the cyclic β-pleated gramicidin S (GS), and the α-helical PGLa. (19)F NMR was used to screen their molecular alignment in various model membranes over a wide range of temperatures. Both peptides were found to respond to the phase state and composition of these different samples in a similar way. In phosphatidylcholines, both peptides first bind to the bilayer surface. Above a certain threshold concentration they can re-align and immerse more deeply into the hydrophobic core, which presumably involves oligomerization. Re-alignment is most favorable around the lipid chain melting temperature, and also promoted by decreasing bilayer thickness. The presence of anionic lipids has no influence in fluid membranes, but in the gel phase the alignment states are more complex. Unsaturated acyl chains and other lipids with intrinsic negative curvature prevent re-alignment, hence GS and PGLa do not insert into mixtures resembling bacterial membranes, nor into bacterial lipid extracts. Cholesterol, which is present in high concentrations in animal membranes, even leads to an expulsion of the peptides from the bilayer and prevents their binding altogether. However, a very low cholesterol content of 10% was found to promote binding and re-alignment of both peptides. Overall, these findings show that the ability of amphiphilic peptides to re-align and immerse into a membrane is determined by the physico-chemical properties of the lipids, such as spontaneous curvature. This idea is reinforced by the remarkably similar behavior observed here for two structurally unrelated molecules (with different conformation, size, shape, charge), which further suggests that their activity at the membrane level is largely governed by the properties of the constituent lipids, while the selectivity towards different cell types is additionally ruled by electrostatic attraction between peptide and cell surface. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

[Progress in the study of some important natural bioactive cyclopeptides]

Natural cyclopeptides are hot spots in chemical and pharmaceutical fields because of the wide spreading bio-resources, complex molecular structures and various bioactivities. Bio-producers of cyclopeptides distribute over almost every kingdom from bacteria to plants and animals. Many cyclopeptides contain non-coded amino acids and non-pepditic bonds. Most exciting characteristic of cyclopeptides is a range of interesting bioactivities such as antibiotics gramicidin-S (2), vancomycin (3) and daptomycin (4), immunosuppressive cyclosporin-A (1) and astin-C (8), and anti-tumor aplidine (5), RA-V (6) and RA-VII (7). Compounds 1-4 are being used in clinics; compounds 5-8 are in the stages of clinical trial or as a candidate for drug research. In this review, the progress in chemical and bioactive studies on these important natural bioactive cyclopeptides 1-8 are introduced, mainly including discovery, bioactivity, mechanism, QSAR and synthesis.

Surface modified silver selinide nanoparticles as extracting probes to improve peptide/protein detection via nanoparticles-based liquid phase microextraction coupled with MALDI mass spectrometry

We report the first use of functionalized Ag(2)Se nanoparticles (NPs) as effective extracting probes for NPs-based liquid-phase microextraction (NPs-LPME) to analyze hydrophobic peptides and proteins from biological samples (urine and plasma) and soybean in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Surface modified functional groups such as octadecanethiol (ODT) and 11-mercaptoundecanoic acid (MUA) on Ag(2)Se NPs were found to play an important role for efficient extraction of peptides and proteins from test samples through hydrophobic interactions. The peptides can be efficiently extracted using functionalized Ag(2)Se NPs as extracting probes in the presence of high concentration of matrix interferences such as 4M urea, 0.5% Triton X-100 and 3% NaCl. Ag(2)Se@ODT NPs have shown better extraction efficiency and detection sensitivity for peptides than Ag(2)Se@MUA NPs, bare Ag(2)Se NPs and conventional MALDI-MS. The LODs are 20-68 nM for valinomycin and 100-180 nM for gramicidin D using Ag(2)Se@ODT NPs-LPME in the MALDI-MS. The current approach is highly sensitive and the target analytes can be effectively isolated without sample loss and efficiently analyzed in MALDI-MS.

Swelling-induced Ca²+ influx and K+ efflux in American alligator erythrocytes

The American alligator can hibernate during winter, which may lead to osmotic imbalance because of reduced kidney function and lack of food consumption during this period. Accordingly, we hypothesized that their red blood cells would have a well-developed regulatory volume decrease (RVD) to cope with the homeostatic challenges associated with torpor. Osmotic fragility was determined optically, mean cell volume was measured by electronic sizing, and changes in intracellular Ca²+ concentration were visualized using fluorescence microscopy and fluo-4-AM. Osmotic fragility increased and the ability to regulate volume was inhibited when extracellular Na+ was replaced with K+, or when cells were exposed to the K+ channel inhibitor quinine, indicating a requirement of K+ efflux for RVD. Addition of the ionophore gramicidin to the extracellular medium decreased osmotic fragility and also potentiated volume recovery, even in the presence of quinine. In addition, hypotonic shock (0.5 x Ringer) caused an increase in cytosolic Ca²+, which resulted from Ca²+ influx because it was not observed when extracellular Ca²+ was chelated with EGTA (ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid). Furthermore, cells loaded with BAPTA-AM (1,2-bis(2-aminophenoxymethyl)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester) or exposed to a low Ca²+-EGTA hypotonic Ringer had a greater osmotic fragility and also failed to recover from cell swelling, indicating that extracellular Ca²+ was needed for RVD. Gramicidin reversed the inhibitory effect of low extracellular Ca²+. Finally, and surprisingly, the Ca²+ ionophore A23187 increased osmotic fragility and inhibited volume recovery. Taken together, our results show that cell swelling activated a K+ permeable pathway via a Ca²+-dependent mechanism, and this process mediated K+ loss during RVD.

On resin amino acid side chain attachment strategy for the head to tail synthesis of new glutamine containing gramicidin-S analogs and their antimicrobial activity

The alarming increase in infections caused by multiple drug resistant bacteria including methicillin-resistant Staphylococcus aureus has prompted a desperate search for new antimicrobials. Augmenting the discoveries of completely new scaffolds with antimicrobial activity are efforts aimed at modifying existing molecules to optimize activity or reduce toxicity. We report herein the parallel solid-phase synthesis of analogues of the cationic antimicrobial peptide gramicidin S (GS) using amino acid side chain attachment strategy. The ornithine (Orn) residues were replaced by glutamine (Gln) and the aromatic D-phenylalanine (Phe) were replaced by different aromatic D-amino acids. Additional Gln containing GS analogues with all the possible combinations of the hydrophobic amino acids valine and leucine were also synthesized. In this work we also report the antibacterial activity of these analogs against several clinically-important drug-resistant Gram-positive and Gram-negative pathogens.

Development of light response and GABAergic excitation-to-inhibition switch in zebrafish retinal ganglion cells

The zebrafish retina has been an important model for studying morphological development of neural circuits in vivo. However, its functional development is not yet well understood. To investigate the functional development of zebrafish retina, we developed an in vivo patch-clamp whole-cell recording technique in intact zebrafish larvae. We first examined the developmental profile of light-evoked responses (LERs) in retinal ganglion cells (RGCs) from 2 to 9 days post-fertilization (dpf). Unstable LERs were first observed at 2.5 dpf. By 4 dpf, RGCs exhibited reliable light responses. As the GABAergic system is critical for retinal development, we then performed in vivo gramicidin perforated-patch whole-cell recording to characterize the developmental change of GABAergic action in RGCs. The reversal potential of GABA-induced currents (E(GABA)) in RGCs gradually shifted from depolarized to hyperpolarized levels during 2-4 dpf and the excitation-to-inhibition (E-I) switch of GABAergic action occurred at around 2.5 dpf when RGCs became light sensitive. Meanwhile, GABAergic transmission upstream to RGCs also became inhibitory by 2.5 dpf. Furthermore, down-regulation of the K(+)/Cl() co-transporter (KCC2) by the morpholino oligonucleotide-based knockdown approach, which shifted RGC E(GABA) towards a more depolarized level and thus delayed the E-I switch by one day, postponed the appearance of RGC LERs by one day. In addition, RGCs exhibited correlated giant inward current (GICs) during 2.5-3.5 dpf. The period of GICs was shifted to 3-4.5 dpf by KCC2 knockdown. Taken together, the GABAergic E-I switch occurs coincidently with the emergence of light responses and GICs in zebrafish RGCs, and may contribute to the functional development of retinal circuits.

Mitochondria as a target in treatment

Mitochondria are key organelles that perform essential cellular functions and play pivotal roles in cell death and survival signaling. Hence, they represent an attractive target for drugs to treat metabolic, degenerative, and hyperproliferative diseases. Targeting mitochondria with organelle-specific agents or prodrugs has proven to be an effective therapeutic strategy. More specifically, controlling the cellular ROS balance via selective delivery of an antioxidant "payload" into mitochondria is an elegant emerging therapeutic concept. Herein, we review the recent medicinal chemistry and clinical data of these exploratory strategies, which should point the way for future generations of therapeutics.

Membrane damage elicits an immunomodulatory program in Staphylococcus aureus

The Staphylococcus aureus HrtAB system is a hemin-regulated ABC transporter composed of an ATPase (HrtA) and a permease (HrtB) that protect S. aureus against hemin toxicity. S. aureus strains lacking hrtA exhibit liver-specific hyper-virulence and upon hemin exposure over-express and secrete immunomodulatory factors that interfere with neutrophil recruitment to the site of infection. It has been proposed that heme accumulation in strains lacking hrtAB is the signal which triggers S. aureus to elaborate this anti-neutrophil response. However, we report here that S. aureus strains expressing catalytically inactive HrtA do not elaborate the same secreted protein profile. This result indicates that the physical absence of HrtA is responsible for the increased expression of immunomodulatory factors, whereas deficiencies in the ATPase activity of HrtA do not contribute to this process. Furthermore, HrtB expression in strains lacking hrtA decreases membrane integrity consistent with dysregulated permease function. Based on these findings, we propose a model whereby hemin-mediated over-expression of HrtB in the absence of HrtA damages the staphylococcal membrane through pore formation. In turn, S. aureus senses this membrane damage, triggering the increased expression of immunomodulatory factors. In support of this model, wildtype S. aureus treated with anti-staphylococcal channel-forming peptides produce a secreted protein profile that mimics the effect of treating ΔhrtA with hemin. These results suggest that S. aureus senses membrane damage and elaborates a gene expression program that protects the organism from the innate immune response of the host.

Staphylococcus aureus infects almost every tissue within the human body utilizing a range of virulence factors to combat host defenses. The expression of these virulence factors is a tightly regulated process; however, the signals sensed by S. aureus during infection remain elusive. It has been hypothesized that heme toxicity is a signal sensed by S. aureus during infection. This hypothesis is based on the observation that S. aureus mutants which are incapable of relieving heme-toxicity due to inactivation of the ATPase HrtA elicit an immunomodulatory program that interferes with neutrophil recruitment to the site of infection. In keeping with this, S. aureus hrtA mutants exhibit liver-specific hypervirulence. Herein, we provide evidence for an alternative model to explain the hypervirulent phenotype of S. aureus ΔhrtA. We demonstrate that instead of accumulation of heme toxicity being the trigger for the observed immunomodulatory program, dysregulated pore formation caused by the HrtB permease triggers the anti-neutrophil response. In support of this model, over-expression of HrtB in wildtype S. aureus or exposing S. aureus to channel-forming antimicrobial peptides induces a similar immunomodulatory program. Our work provides evidence that S. aureus senses membrane damage and induces an immunomodulatory circuit that helps the pathogen evade immune-mediated clearance.

In vitro antimicrobial activity of alpha-melanocyte stimulating hormone against major human pathogen Staphylococcus aureus

Alpha-melanocyte stimulating hormone (alpha-MSH) is an endogenous anti-inflammatory peptide reported to possess antimicrobial properties, however their role as antibacterial peptides is yet to be established. In the present study, we examined in vitro antibacterial activity of alpha-MSH against S. aureus strain ISP479C and several methicillin-sensitive (MSSA) and methicillin-resistant (MRSA) S. aureus strains. Antibacterial activity was examined by varying several parameters, viz., bacterial cell densities, growth phase, pH, salt concentration, and temperature. Antibacterial activity was also examined in complex biomatrices of rat whole blood, plasma and serum as well as in biofilm form of bacteria. Our results showed that alpha-MSH possessed significant and rapid antibacterial activity against all the studied strains including MRSA (84% strains were killed on exposure to 12 microM of alpha-MSH for 2h). pH change from 7.4 to 4 increased alpha-MSH staphylocidal activity against ISP479C by 21%. Antibacterial activity of alpha-MSH was dependent on bacterial cell density and independent of growth phase. Moreover, antimicrobial activity was retained when alpha-MSH was placed into whole blood, plasma, and serum. Most importantly, alpha-MSH exhibited antibacterial activity against staphylococcal biofilms. Multiple membrane permeabilization assays suggested that membrane damage was, at least in part, a major mechanism of staphylocidal activity of alpha-MSH. Collectively the above findings suggest that alpha-MSH could be a promising candidate of a novel class of antimicrobial agents.

Effect of membrane composition on antimicrobial peptides aurein 2.2 and 2.3 from Australian southern bell frogs

The effects of hydrophobic thickness and the molar phosphatidylglycerol (PG) content of lipid bilayers on the structure and membrane interaction of three cationic antimicrobial peptides were examined: aurein 2.2, aurein 2.3 (almost identical to aurein 2.2, except for a point mutation at residue 13), and a carboxy C-terminal analog of aurein 2.3. Circular dichroism results indicated that all three peptides adopt an alpha-helical structure in the presence of a 3:1 molar mixture of 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPC/DMPG), and 1:1 and 3:1 molar mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPC/POPG). Oriented circular dichroism data for three different lipid compositions showed that all three peptides were surface-adsorbed at low peptide concentrations, but were inserted into the membrane at higher peptide concentrations. The (31)P solid-state NMR data of the three peptides in the DMPC/DMPG and POPC/POPG bilayers showed that all three peptides significantly perturbed lipid headgroups, in a peptide or lipid composition-dependent manner. Differential scanning calorimetry results demonstrated that both amidated aurein peptides perturbed the overall phase structure of DMPC/DMPG bilayers, but perturbed the POPC/POPG chains less. The nature of the perturbation of DMPC/DMPG bilayers was most likely micellization, and for the POPC/POPG bilayers, distorted toroidal pores or localized membrane aggregate formation. Calcein release assay results showed that aurein peptide-induced membrane leakage was more severe in DMPC/DMPG liposomes than in POPC/POPG liposomes, and that aurein 2.2 induced higher calcein release than aurein 2.3 and aurein 2.3-COOH from 1:1 and 3:1 POPC/POPG liposomes. Finally, DiSC(3)5 assay data further delineated aurein 2.2 from the others by showing that it perturbed the lipid membranes of intact S. aureus C622 most efficiently, whereas aurein 2.3 had the same efficiency as gramicidin S, and aurein 2.3-COOH was the least efficient. Taken together, these data show that the membrane interactions of aurein peptides are affected by the hydrophobic thickness of the lipid bilayers and the PG content.

Proline residue-modified polycationic analogs of gramicidin S with high antibacterial activity against both Gram-positive and Gram-negative bacteria and low hemolytic activity*

Novel polycationic analogs of the cyclic decapeptide antibiotic, gramicidin S, possessing NH(2), D/L-Phe-NH or L-Lys-NH groups at the 4alpha- or 4beta-positions of the L-Pro residues, were synthesized. While L-Pro(4alpha/beta-NH(2))-containing analogs exhibited much weaker antibacterial activity, the D/L-Phe and L-Lys-substituted analogs exhibited higher antibacterial activity against Gram-negative bacteria than the parent gramicidin S. All of these additional amino group-containing analogs showed substantially reduced toxicity against human blood cells.

Bacteriophage formation without bacterial growth; the effect of iodoacetate, fluoride, gramicidin, and azide on the formation of bacteriophage

1. Iodoacetate, fluoride, and azide have been found to prevent the formation of phage and to inhibit the synthesis of ATP by Staphylococcus muscae. It is suggested that energy-rich phosphate is needed for the synthesis of phage. 2. Gramicidin prevented the formation of phage. 3. No differences were found between normal bacteria and phage-infected bacteria in the inorganic phosphate, adenosinetriphosphate, ribonucleic acid, and desoxyribonucleic acid content of the cells. 4. The mechanism of phage formation is discussed.

Interaction of gramicidin S and its aromatic amino-acid analog with phospholipid membranes

To investigate the mechanism of interaction of gramicidin S-like antimicrobial peptides with biological membranes, a series of five decameric cyclic cationic beta-sheet-beta-turn peptides with all possible combinations of aromatic D-amino acids, Cyclo(Val-Lys-Leu-D-Ar1-Pro-Val-Lys-Leu-D-Ar2-Pro) (Ar identical with Phe, Tyr, Trp), were synthesized. Conformations of these cyclic peptides were comparable in aqueous solutions and lipid vesicles. Isothermal titration calorimetry measurements revealed entropy-driven binding of cyclic peptides to POPC and POPE/POPG lipid vesicles. Binding of peptides to both vesicle systems was endothermic-exceptions were peptides containing the Trp-Trp and Tyr-Trp pairs with exothermic binding to POPC vesicles. Application of one- and two-site binding (partitioning) models to binding isotherms of exothermic and endothermic binding processes, respectively, resulted in determination of peptide-lipid membrane binding constants (K(b)). The K(b1) and K(b2) values for endothermic two-step binding processes corresponded to high and low binding affinities (K(b1) >or= 100 K(b2)). Conformational change of cyclic peptides in transferring from buffer to lipid bilayer surfaces was estimated using fluorescence resonance energy transfer between the Tyr-Trp pair in one of the peptide constructs. The cyclic peptide conformation expands upon adsorption on lipid bilayer surface and interacts more deeply with the outer monolayer causing bilayer deformation, which may lead to formation of nonspecific transient peptide-lipid porelike zones causing membrane lysis.

Hemigramicidin-TEMPO conjugates: novel mitochondria-targeted antioxidants

Reactive oxygen species (ROS) are reactive, partially reduced derivatives of molecular oxygen. ROS are important in the pathogenesis of a wide range of acute pathologic processes, including ischemia/reperfusion injury, sepsis, and shock. Accordingly, effective ROS scavengers might be useful therapeutic agents for these conditions. Since mitochondria are the primary sites for ROS production within cells, it seems reasonable that targeting ROS scavengers to these organelles could be a particularly effective strategy. Indeed, a number of compounds or classes of compounds have been described that are based on this concept. One approach consists of coupling a payload--the portion of the molecule with ROS-scavenging activities--to a targeting moiety--the portion of the molecule that promotes selective accumulation within mitochondria. For example, the payload portion of XJB-5-131 consists of a stable nitroxide radical, which has been extensively investigated as a cytoprotective agent in a number of experimental models of oxidative stress. The targeting portion of XJB-5-131 consists of a portion of the membrane-active cyclopeptide antibiotic, gramicidin S. The gramicidin segment was used to target the nitroxide payload to mitochondria because antibiotics of this type have a high affinity for bacterial membranes and because of the close relationship between bacteria and mitochondria. In a rat model of hemorrhagic shock, delayed treatment with XJB-5-131 has been shown to prolong survival time in the absence of resuscitation with blood or a large volume of crystalloid fluid. Compounds like XJB-5-131 warrant further evaluation for the treatment of hemorrhagic shock as well as other acute conditions associated with increased mitochondrial production of ROS.

[Effect of copper ions on the light-induced proton transfer in spinach chloroplasts]

It was shown that the light-dependent proton uptake by a suspension of isolated chloroplasts was completely inhibited in the presence of 30-50 microM Cu2+ ions at the 0.1-0.3 Cu2+/Chl ratio. At the same time, the rate of photosynthetic oxygen evolution in the presence of 30-200 microM CuSO4 was reduced by no more than 20-30% of control and up to 50% of the control DeltapH value was retained. The results allow us to suppose that, in the presence of copper ions: 1) electron transport in PS2 is inhibited at the level of the secondary quinone acceptor Q(B) whose photoreduction is accompanied by proton uptake from external medium; and 2) an alternative pathway of electron transfer to terminal acceptor is activated, which provides the photooxidation of water and the formation of transmembrane proton gradient.

Proton transport through influenza A virus M2 protein reconstituted in vesicles

Influenza A virus M2 protein is known to form acid-activated, proton-selective, amantadine-sensitive channels. We directly measured proton uptake in vesicles containing reconstituted M2 by monitoring external pH after addition of valinomycin to vesicles with 100-fold-diluted external [K(+)]. External pH typically increased by a few tenths of a pH unit over a few minutes after valinomycin addition, but proton uptake was not significantly altered by acidification. Under neutral conditions, external addition of 1 mM amantadine produced a reduction in flux consistent with randomly ordered channels; however, experimental variation is high with this method and the block was not statistically significant. Amantadine block was reduced at pH 5.4. In accord with Lin and Schroeder's study of reconstituted M2 using a pH-sensitive dye to monitor intravesicular pH, we conclude that bath pH weakly affects or does not significantly affect proton flow in the pH range 5.4-7.0 for the reconstituted system, contrary to results from electrophysiological studies. Theoretical analysis of the relaxation to Donnan equilibrium utilized for such vesicle uptake assays illuminates the appropriate timescale of the initial slope and an important limitation that must be placed on inferences about channel ion selectivity. The rise in pH over 10 s after ionophore addition yielded time-averaged single-channel conductances of 0.35 +/- 0.20 aS and 0.72 +/- 0.42 aS at pH 5.4 and 7.0, respectively, an order of magnitude lower than previously reported in vesicles. Assuming complete membrane incorporation and tetramerization of the reconstituted protein, such a low time-averaged conductance in the face of previously observed single-channel conductance (6 pS at pH 3) implies an open channel probability of 10(-6)-10(-4). Based on leakage of potassium from M2-containing vesicles, compared to protein-free vesicles, we conclude that M2 exhibits approximately 10(7) selectivity for hydrogen over potassium.

The ability of Aneurinibacillus migulanus (Bacillus brevis) to produce the antibiotic gramicidin S is correlated with phenotype variation

Phenotype instability of bacterial strains can cause significant problems in biotechnological applications, since industrially useful properties may be lost. Here we report such degenerative dissociation for Aneurinibacillus migulanus (formerly known as Bacillus brevis) an established producer of the antimicrobial peptide gramicidin S (GS). Phenotypic variations within and between various strains maintained in different culture collections are demonstrated. The type strain, ATCC 9999, consists of six colony morphology variants, R, RC, RP, RT, SC, and SP, which were isolated and characterized as pure cultures. Correlations between colony morphology, growth, GS production, spore formation, and resistance to their own antimicrobial peptide were established in this study. We found the original R form to be the best producer, followed by RC, RP, and RT, while SC and SP yielded no GS at all. Currently available ATCC 9999(T) contains only 2% of the original R producer and is dominated by the newly described phenotypes RC and RP. No original R form is detected in the nominally equivalent strain DSM 2895(T) (=ATCC 9999(T)), which grows only as SC and SP phenotypes and has thus completely lost its value as a peptide producer. Two other strains from the same collection, DSM 5668 and DSM 5759, contain the unproductive SC variant and the GS-producing RC form, respectively. We describe the growth and maintenance conditions that stabilize certain colony phenotypes and reduce the degree of degenerative dissociation, thus providing a recommendation for how to revert the nonproducing smooth phenotypes to the valuable GS-producing rough ones.

Membrane permeabilization of a mammalian neuroendocrine cell type (PC12) by the channel-forming peptides zervamicin, alamethicin, and gramicidin

Zervamicin IIB (ZER) is a 16-mer peptaibol that produces voltage-dependent conductances in artificial membranes, a property considered responsible for its antimicrobial activity to mainly Gram-positive microorganisms. In addition, ZER appears to inhibit the locomotor activity of the mouse (see elsewhere in this Issue), probably by affecting the brain. To examine whether the electrophysiological properties of the neuronal cells of the central neural system might be possibly influenced by the pore forming ZER, the present study was undertaken as a first attempt to unravel the molecular mechanism of this biological activity. To this end, membrane permeabilization of the neuron-like rat pheochromocytoma cell (PC12) by the channel-forming ZER was studied with the whole-cell patch-clamp technique, and compared with the permeabilizations of the well-known voltage-gated peptaibol alamethicin F50/5 (ALA) and the cation channel-forming peptide-antibiotic gramicidin D (GRAM). While 1 muM GRAM addition to PC12 cells kept at a membrane potential V(m)=0 mV causes an undelayed gradual increase of a leak conductance with a negative reversal potential of ca. -24 mV, ZER and ALA are ineffective at that concentration and potential. However, if ZER and ALA are added in 5-10 microM concentrations while V(m) is kept at -60 mV, they cause a sudden and strong permeabilization of the PC12 cell membrane after a delay of 1-2 min, usually leading to disintegrating morphology changes of the patched cell but not of the surrounding cells of the culture at that time scale. The zero reversal potential of the established conductance is consistent with the known aselectivity of the channels formed. This sudden permeabilization does not occur within 10-20 min at V(m)=0 mV, in accordance with the known voltage dependency of ZER and ALA channel formation in artificial lipid membranes. The permeabilizing action of these peptaibols on the culture as a whole is further supported by K(+)-release measurements from a PC12 suspension with a K(+)-selective electrode. Further analysis suggested that the permeabilizing action is associated with extra- or intracellular calcium effects, because barium inhibited the permeabilizing effects of ZER and ALA. We conclude, for the membrane of the mammalian neuron-like PC12 cell, that the permeabilizing effects of the peptides ZER and ALA are different from those of GRAM, consistent with earlier studies of these peptides in other (artificial) membrane systems. They are increased by cis-positive membrane potentials in the physiological range and may include calcium entry into the PC12 cell.

Biofilms protect Mycoplasma pulmonis cells from lytic effects of complement and gramicidin

The length of the tandem repeat region of the Vsa protein of Mycoplasma pulmonis has previously been shown to modulate the susceptibility of mycoplasmas to killing by complement: cells that produce a short form of the Vsa protein are highly sensitive, and cells producing the long Vsa protein are resistant. In contrast to their differing susceptibilities to complement, the mycoplasmas were highly sensitive to gramicidin irrespective of the length of the Vsa protein produced. We show here that when encased within a biofilm, cells of M. pulmonis producing a short form of the Vsa protein were more resistant to complement and gramicidin than mycoplasmas that were dispersed. The resistance appeared to be localized to those mycoplasmas within tower structures of the biofilms. Biofilm formation may be a mechanism that protects mycoplasmas from host immunity.

Tri-adcortyl ointment ear dressing in myringoplasty: an analysis of outcome

In a retrospective study of patients who had undergone myringoplasty at our department within a 12-month period, we assessed the graft take rate using tri-adcortyl ointment (TAO) as ear dressing. Data including age, site and size of perforation, grade of surgeon, surgical approach, use of postoperative ear dressings, complications and audiometric outcome was collected from the patient notes and analysed. The overall success rate of the operation (with success being defined as an intact tympanic membrane at 6 months) was noted. Seventy-seven patients were operated, but data were complete on 64 patients and these constituted the study population. TAO was used in 95% of the patients and BIPP gauze pack in the remaining 5%. Age ranged from 8 to 63 years (mean 34 years). Mean follow-up period was 13 months. Most of the operations (72.88%) were carried out by consultants with a success rate of 89% and the remaining patients were operated by trainees with a success rate of 88% (P = 1.000). The overall success rate was 89, 90.16% for TAO and 66.66% (2 out of 3) for BIPP (P = 0.298), 95% for small and 86% for subtotal perforations (P = 0.573), 85% for anterior and 100% for posterior perforations (P = 0.240), 91% for adult patients and 88% for children (P = 1.000). TAO is a suitable ear dressing in myringoplasty. Routine use of TAO did not affect the success rate of myringoplasty at our centre.

Assessing the data quality in predictive toxicology using a panel of cell lines and cytotoxicity assays

In vitro cell viability assays have a central role in predictive toxicology, both in assessing acute toxicity of chemicals and as a source of experimental data for in silico methods. However, the quality of in vitro toxicity databanks fluctuates dramatically because information they contain is obtained under varying conditions and in different laboratories. The aim of this study was to identify the factors responsible for these deviations and thus the quality of the data extracted for predictive toxicology. Three cell viability assays measuring LDH leakage, WST-1 reduction, and intracellular ATP were compared in an automated environment using four mammalian cell lines: Caco-2, Calu-3, Huh-7, and BHK. Using four standard compounds--polymyxin B, gramicidin, 5-fluorouracil, and camptothecin--a significant lack of sensitivity in LDH assay compared with the other assays was observed. Because the viability IC(50) values for the standards were similar among the cell lines, the biochemical characteristics of different cell lines seem to play only a minor role, with an exception being the hepatocellular Huh-7 cell line. Toxicity assessment of new 1,2,4-triazoles revealed significant differences in their toxic potential, and the results indicate the same sensitivity profile among the assays as observed with the standard compounds. Overall, it can be argued that the assay selection is the most important factor governing the uniform quality of the data obtained from in vitro cell viability assays.

Presence of a Na+-stimulated P-type ATPase in the plasma membrane of the alkaliphilic halotolerant cyanobacterium Aphanothece halophytica

Aphanothece cells could take up Na(+) and this uptake was strongly inhibited by the protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Cells preloaded with Na(+) exhibited Na(+) extrusion ability upon energizing with glucose. Na(+) was also taken up by the plasma membranes supplied with ATP and the uptake was abolished by gramicidin D, monensin or Na(+)-ionophore. Orthovanadate and CCCP strongly inhibited Na(+) uptake, whereas N, N'-dicyclohexylcarbodiimide (DCCD) slightly inhibited the uptake. Plasma membranes could hydrolyse ATP in the presence of Na(+) but not with K(+), Ca(2+) and Li(+). The K(m) values for ATP and Na(+) were 1.66+/-0.12 and 25.0+/-1.8 mM, respectively, whereas the V(max) value was 0.66+/-0.05 mumol min(-1) mg(-1). Mg(2+) was required for ATPase activity whose optimal pH was 7.5. The ATPase was insensitive to N-ethylmaleimide, nitrate, thiocyanate, azide and ouabain, but was substantially inhibited by orthovanadate and DCCD. Amiloride, a Na(+)/H(+) antiporter inhibitor, and CCCP showed little or no effect. Gramicidin D and monensin stimulated ATPase activity. All these results suggest the existence of a P-type Na(+)-stimulated ATPase in Aphanothece halophytica. Plasma membranes from cells grown under salt stress condition showed higher ATPase activity than those from cells grown under nonstress condition.

Insulin increases the potency of glycine at ionotropic glycine receptors

The mechanisms by which insulin modulates neuronal plasticity and pain processes remain poorly understood. Here we report that insulin rapidly increases the function of glycine receptors in murine spinal neurons and recombinant human glycine receptors expressed in human embryonic kidney cells. Whole-cell patch-clamp recordings showed that insulin reversibly enhanced current evoked by exogenous glycine and increased the amplitude of spontaneous glycinergic miniature inhibitory postsynaptic currents recorded in cultured spinal neurons. Insulin (1 microM) also shifted the glycine concentration-response plot to the left and reduced the glycine EC(50) value from 52 to 31 microM. Currents evoked by a submaximal concentration of glycine were increased to approximately 140% of control. The glycine receptor alpha subunit was sufficient for the enhancement by insulin because currents from recombinant homomeric alpha(1) receptors and heteromeric alpha(1)beta receptors were both increased. Insulin acted at the insulin receptor via pathways dependent on tyrosine kinase and phosphatidylinositol 3 kinase because the insulin effect was eliminated by the insulin receptor antagonist, hydroxy-2-naphthalenylmethylphosphonic acid trisacetoxymethyl ester, the tyrosine kinase inhibitor lavendustin A, and the phosphatidylinositol 3 kinase antagonist wortmannin. Together, these results show that insulin has a novel regulatory action on the potency of glycine for ionotropic glycine receptors.

[Cell wall components in Staphylococcus aureus with double resistance to gramicidin S and actinomycin D]

Cell walls of Staphylococcus aureus R9/80 resistant to gramicidin S and actinomycin D were investigated. The strain was isolated after passages of a previously isolated strain of S. aureus with resistance to gramicidin and definite changes in the cell walls, a medium with increasing concentrations of actinomycin being used for the passages. The data on the study of the cell walls of the strain with the double resistance were compared with the results of the investigation of the cell walls of the strain susceptible to gramicidin, the gramicidin resistant strain (initial for strain R9/80) and the actinomycin adapted strain that also showed changes in the cell walls. The cell walls of the resistant strains had no significant changes in the peptidoglycane and glucosamine levels, as well as in the peptidoglycane amino acid composition. Teichoic acids of all the strains had different levels of substitution of ribite by D-alanine (a factor influencing the negative charge of teichoic acids and the wall at large). It was noted that all the strains resistant to the tested antibiotics had lower levels of teichoic acids in the cell walls. The resistant cells showed some increase of the lipid component in the walls: from 1.6% in the susceptible strain to 2.1-2.9% in the resistant cells. The main trend of the changes in the resistance development was revealed to be the thickening of the cell wall and its consolidation. The development of resistance to gramicidin, actinomycin and to both the antibiotics provoked respectively a 2.4-, 4- and 5.4-fold increase of the content of the main cell component. i.e. peptidoglycane in the cell biomass. The barrier role of the cell walls in the resistant strains and their ability to bind the antibiotic is discussed.

The involvement of Na+/K(+)-ATPase in the development of platelet procoagulant response

In circulation, platelets may come into contact with both exogenous (cardiac glycoside treatment) and endogenously produced inhibitors of Na+/K(+)-ATPase. We examined whether blocking of platelet Na+/K(+)-ATPase by ouabain results in generation of procoagulant activity. It was shown that an in vitro treatment of platelets with ouabain (20-200 microM for 20 to 60 min) is associated with an intracellular accumulation of sodium ([Na+](i)), generation of a weak calcium signal, and expression of procoagulant activity. The ouabain-induced procoagulant response was dose- and time-related, less pronounced than that evoked by collagen and similar to that produced by gramicidin, not affected by EDTA or aspirin, and strongly reduced in the absence of extracellular Na+ or by hyperosmolality. Flow cytometry studies revealed that ouabain treatment results in a unimodal left shift in the forward and side scatter of the entire platelet population indicating morphological changes of the plasma membrane. The shift was dose related, weaker than that evoked by collagen and similar to that produced by gramicidin. Ouabain-treated platelets express phosphatidylserine (PS). The ouabain-evoked PS expression was dose- and time-dependent, weaker than that produced by collagen and similar to that evoked by gramicidin. Electronic cell sizing measurements showed a dose-dependent increase in mean platelet volume upon treatment with ouabain. Hypoosmotically-evoked platelet swelling resulted in the appearance of procoagulant activity. Thromboelastography measurements indicate that, in whole blood, nanomolar (50-1000 nM, 15 min) concentrations of ouabain significantly accelerate the rate of clot formation initiated by contact and high extracellular concentration of calcium. We conclude that inefficiently operating platelet Na+/K(+)-ATPase results in a rise in [Na+](i). An increase in [Na+](i) and the swelling associated with it may produce PS exposure and a rise in membrane curvature leading to the generation of a procoagulant activity.

Synthesis of Low-Hemolytic Antimicrobial Dehydropeptides Based on Gramicidin S

The synthesis and biological activity of a novel cyclic beta-sheet-type antimicrobial dehydropeptide based on gramicidin S (GS) is described. The GS analogue, containing two (Z)-(beta-3-pyridyl)-alpha,beta-dehydroalanine (DeltaZ3Pal) residues at the 4 and 4' positions (2), was synthesized by solution-phase methodologies using Boc-Leu-DeltaZ3Pal azlactone. Analogue 2 exhibited high antimicrobial activity against Gram-positive bacteria and had much lower hemolytic activity than wild-type GS and the corresponding (Z)-alpha,beta-dehydrophenylalanine (DeltaZPhe) analogue (1).

Estimate of the chloride concentration in a central glutamatergic terminal: a gramicidin perforated-patch study on the calyx of Held

The function of presynaptic terminals is regulated by intracellular Cl-, the levels of which modify vesicular endocytosis and transmitter refilling and mediate the effects of presynaptic ligand-gated Cl- channels. Nevertheless, the concentration of Cl- in a central nerve terminal is unknown, and it is unclear whether terminals can regulate Cl- independently of the soma. Using perforated-patch recording in a mammalian synapse, we found that terminals accumulate Cl- up to 21 mm, between four and five times higher than in their parent cell bodies. Changing [Cl-] did not alter vesicular glutamate content in intact terminals, unlike in vitro experiments. Thus, glutamatergic terminals maintain an elevated Cl- concentration without compromising synaptic transmission.

Oxygen reduction in chloroplast thylakoids results in production of hydrogen peroxide inside the membrane

Hydrogen peroxide production in isolated pea thylakoids was studied in the presence of cytochrome c to prevent disproportionation of superoxide radicals outside of the thylakoid membranes. The comparison of cytochrome c reduction with accompanying oxygen uptake revealed that hydrogen peroxide was produced within the thylakoid. The proportion of electrons from water oxidation participating in this hydrogen peroxide production increased with increasing light intensity, and at a light intensity of 630 micromol quanta m(-2) s(-1) it reached 60% of all electrons entering the electron transport chain. Neither the presence of a superoxide dismutase inhibitor, potassium cyanide or sodium azide, in the thylakoid suspension, nor unstacking of the thylakoids appreciably affected the partitioning of electrons to hydrogen peroxide production. Also, osmolarity-induced changes in the thylakoid lumen volume, as well as variation of the lumen pH induced by the presence of Gramicidin D, had negligible effects on such partitioning. The flow of electrons participating in lumen hydrogen peroxide production was found to be near 10% of the total electron flow from water. It is concluded that a considerable amount of hydrogen peroxide is generated inside thylakoid membranes, and a possible mechanism, as well as the significance, of this process are discussed.

A novel, antimicrobially active analog of gramicidin S without amphiphilic conformation

A novel gramicidin S analog, cyclo(-Val-Leu-Leu-Orn-Leu-D-Phe-Pro-)2, was synthesized, its antibiotic activity compared with gramicidin S and shown to be as potent as gramicidin S when compared with the susceptibility toward five Gram-positive microorganisms. It exceeded the activity of gramicidin S against Bacillus megaterium ATCC 19213 by a factor of two. Circular dichroism and NMR data suggested this analog to adopt an antiparallel beta-sheet conformation without amphiphilic character.

Patch-clamp studies in the CNS illustrate a simple new method for obtaining viable neurons in rat brain slices: glycerol replacement of NaCl protects CNS neurons

Viable neurons in brain slices are crucial for electrophysiological studies. The present study describes a new method for obtaining viable cells in several regions of the central nervous system including the ventral tegmental area, the hypothalamus, the periaqueductal grey matter and the spinal cord. The essence of the method was to use a modified artificial cerebrospinal fluid (ACSF) in which all NaCl was replaced initially by equi-osmotic glycerol. This modified glycerol-based ACSF was used during slice preparation. The underlying principle for the modification is to prevent the possible acute neurotoxic effects of passive chloride entry, subsequent cell swelling and lysis. This method significantly increased the live/dead ratio in morphology compared to the normal ACSF or sucrose-base ACSF, in which NaCl was replaced by sucrose. An examination of some electrophysiological and pharmacological properties of the neurons in these preparations, by means of current-clamp and voltage-clamp recordings, revealed similar properties of those neurons obtained with the traditional ACSF method. Due to the increase in the number of viable neurons, the new ACSF increases the productivity of experiments. Based on our data, we propose that this glycerol-based solution may protect CNS neurons.

β-Turn Modified Gramicidin S Analogues Containing Arylated Sugar Amino Acids Display Antimicrobial and Hemolytic Activity Comparable to the Natural Product

This paper describes the design and synthesis of gramicidin S (GS) analogues 10a-c containing arylated sugar amino acids (SAAs) as a replacement of one of the two (D)Phe-Pro beta-turn regions. The cyclic, amphiphilic peptides adopt a beta-sheet conformation featuring an unusual reverse turn induced by the SAAs. The altered turn region induces a slight distortion of the antiparallel beta-sheet, as compared to GS; the overall geometry however closely resembles that of the nonarylated GS analogue 1. GS analogues 10a-c proved to be as active as the parent GS itself as antibacterial agents and are equally efficient in lysing red blood cells. These properties are in sharp contrast to the diminished biological activity displayed by 1. We conclude that the presence of aromaticity in the turn regions of GS derivatives is required for biological activity, whereas the native conformation of the beta-hairpin is not. Our findings may guide future research toward efficient and nonhemolytic GS analogues for combating bacterial infections.

The role of proton motive force in expression of the Staphylococcus aureus cid and lrg operons

The Staphylococcus aureus cidABC and lrgAB operons have been shown to play a key role in the regulation of murein hydrolase activity and cell death in a manner thought to be analogous to bacteriophage-encoded holins and anti-holins respectively. Because of these functions, it has been proposed that the regulation of these operons is tightly controlled and responsive to key metabolic signals. The current study revealed the presence of two overlapping regulatory pathways controlling cidABC and lrgAB expression, one dependent on acetic acid and the other dependent on proton motive force (PMF). The latter pathway was analysed using agents that affect various aspects of the PMF. Gramicidin and carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimicrobial agents that dissipate the DeltapH and membrane potential (DeltaPsi), both enhanced lrgAB expression. Restoration of the PMF by incubation of the bacteria in the presence of glucose restored lrgAB expression back to the uninduced state. In addition, valinomycin, which specifically collapses the DeltaPsi, also induced lrgAB expression. In contrast, nigericin, which dissipates the DeltapH component of the PMF, was found to have a minimal effect on DeltaPsi and lrgAB transcription. Finally, the DeltaPsi-inducible expression of lrgAB was shown to be dependent on the previously characterized LytSR two-component regulatory system that is involved in the regulation of autolysis. The results of this study support a model in which the LytSR regulatory system responds to a collapse in DeltaPsi by inducing the transcription of the lrgAB operon.