Antifungal Amphiphilic Aminoglycosides.

The attachment of alkyl and other hydrophobic groups to traditional antibacterial kanamycins and neomycins creates amphiphilic aminoglycosides with altered antimicrobial properties. In this review, we summarize the discovery of amphiphilic kanamycins that are antifungal, but not antibacterial, and that inhibit the growth of fungi by perturbation of plasma membrane functions. With low toxicities against plant and mammalian cells, they appear to specifically target the fungal plasma membrane. These new antifungal agents offer new options for fighting fungal pathogens and are examples of reviving old drugs to confront new therapeutic challenges.

Voltage-dependent synchronization of gating of syringomycin E ion channels.

Antifungal lipodepsipeptide syringomycin E (SRE) forms two major conductive states in lipid bilayers: "small" and "large". Large SRE channels are cluster of several small ones, demonstrating synchronous opening and closure. To get insight into the mechanism of such synchronization we investigated how transmembrane potential, membrane surface charge, and ionic strength affect the number of small SRE channels synchronously functioning in the cluster. Here, we report that the large SRE channels can be presented as 3-8 simultaneously gating small channels. The increase in the absolute value of the transmembrane potential (from 50 to 200 mV) decreases the number of synchronously gated channels in the clusters. Voltage-dependence of channel synchronization was influenced by the ionic strength of the bathing solution, but not by membrane surface charge. We propose a mechanism for the voltage-dependent cluster behavior that involves a voltage-induced reorientation of lipid dipoles associated with the channel pores.

[Kinetic parameters of single ion channels and stationary conductivities of phytotoxin modified lipid bilayers].

As shown earlier, phytotoxins produced by Pseudomonas syringae pv. syringae form ion channels of "small" and "large" conductance when incorporated into planar lipid membranes. The multilevel conductance is due to cluster organization of the channels (Kaulin et al., 1998; Gurnev et al., 2002). In this study the kinetic parameters of syringomycin E (SRE) and syringostatin A (SSA) channels in negatively charged bilayer lipid membranes were estimated. The average time of open state of the small channels (t(s)(open)) did not depend on transmembrane voltage (in the range of +/- 200 mV). The channel characteristics differed between two phytotoxins: the t(s)(open) for the SRE-channels was much larger than that for SSA-channels. An energetic diagram with two non-conducting states illustrating the formation of the small channel is proposed to explain the voltage independence of the kinetic parameters. The probability for synchronous functioning of small channels with SSA was higher than that with SRE. To analyse the role of the clusters in the biological activities of SRE and SSA, we estimated the cluster contribution to a net transmembrane currents to be 60 and 90%, respectively.

[Interaction between filamentous actin and lipid bilayer causes the increase of syringomycin E channel-forming activity]. / Vzaimodeistvie fibrilliarnogo aktina s lipidnym bisloem vyzyvaet uvelichenie kanaloformennoi aktivnosti siringomitsina E.

The effect of filamentous (F) actin on the channel-forming activity of syringomycin E (SRE) in negatively charged and uncharged bilayer lipid membranes (BLM) was studied. F-actin did not affect the membrane conductance in the absence of SRE. No changes in SRE-induced membrane conductance were observed when the above agents were added to the same side of BLM. However, the opposite side addition of F-actin and SRE provokes a multiple increase in membrane conductance. The similar voltage dependence of membrane conductance, equal values of single channel conductance and the effective gating charge of the channels upon F-actin action suggests that the actin-dependent increase in BLM conductance may result from an increase in the number of opened SRE-channels. BLM conductance kinetics depends on the sequence of SRE and F-actin addition, suggesting that actin-dependent rise of conductance may be induced by BLM structural changes that follow F-actin adsorption. F-actin exerted similar effect on membrane conductance of both negatively charged and uncharged bilayers, as well as on conductance of BLM with high ionic strength bathing solution, suggesting the major role for hydrophobic interactions in F-actin adsorption on lipid bilayer.

Kinetics of opening and closure of syringomycin E channels formed in lipid bilayers.

A cyclic lipodepsipeptide, syringomycin E (SME), incorporated into planar lipid membranes forms two types of channels ("small" and "large") different in their conductance by approximately a factor of six (Biophys. J. 74:2918-2925 (1998)). We analysed the dynamics of the SME-induced transmembrane current under voltage-clamp conditions to clarify the mechanisms of formation of these channels. The voltage-dependent opening/closure of SME channels in lipid bilayers are interpreted in terms of transitions between three types of clusters including 6-7 SME molecules and some lipid molecules. The initial cluster, the precursor of the other two, was in equilibrium with SME monomer molecules at the membrane surface. The other two types of clusters (State 1 and State 2) were formed from the precursor and also during their interconversions (the consecutive-parallel mechanism of transitions). State 1 was a non-conducting state in equilibrium with small channels, which partially determined the ionic conductance of lipid bilayers modified by SME. State 2 corresponded to large SME channels, major contributors to the conductance of a bilayer. The results of the theoretical analysis based on the chemical kinetics concepts were consistent with experimental observations. Such properties of the SME-induced channels as cluster organisation, voltage dependence and the existence of a non-conducting state are all features shared by many ion channels in biological membranes. This makes it possible to use SME channels as a model to study naturally occurring ion channels.

Membrane-permeabilizing activities of cyclic lipodepsipeptides, syringopeptin 22A and syringomycin E from Pseudomonas syringae pv. syringae in human red blood cells and in bilayer lipid membranes.

The pore-forming activities of cyclic lipodepsipeptides (CLPs), syringopeptin 22A (SP22A) and syringomycin E (SRE) were compared on the human red blood cell (RBC) membrane and on bilayer lipid membranes (BLMs). SP22A above a concentration of 4 x 10(5) molecules/cell significantly increased the RBC membrane permeability for 86Rb. With electric current measurements on BLM, it was proved that like SRE, the SP22A formed two types of ion channels in the membrane, small and large, the latter having six times larger conductance and longer dwell time. Both CLPs formed clusters consisting of six small channels, and the channel-forming activity of SP22A is about one order of magnitude higher than that of SRE. A Hill coefficient of 2-3 estimated from the concentration dependence of these CLPs-induced lysis gave a proof of the pore oligomerization on RBCs. Transport kinetic data also confirmed that SP22A pores were oligomers of at least three monomers. While SRE pores were inactivated in time, no pore inactivation was observed with SP22A. The 86Rb efflux through SP22A-treated RBCs approached the tracer equilibrium distribution with a constant rate; a constant integral current was measured on the BLM for as long as 2.5 h as well. The partition coefficient (Kp = 2 x 10(4) l/mol) between the RBC membrane and the extracellular space was estimated for SRE to be at least six times higher than that for SP22A. This finding suggested that the higher ion permeability of the SP22A-treated cells compared to that of SRE was the result of the higher pore-forming activity of SP22A.

Requirement of sphingolipid alpha-hydroxylation for fungicidal action of syringomycin E.

Syringomycin E is an antifungal cyclic lipodepsinonapeptide produced by Pseudomonas syringae pv. syringae. To understand the mechanism of action of syringomycin E, a novel resistant Saccharomyces cerevisiae strain, BW7, was isolated and characterized. Lipid analyses revealed that BW7 contained only the hydrophobic subspecies of sphingolipids that are normally minor components in wild type strains. This aberrant sphingolipid composition was the result of lack of alpha-hydroxylation of the amide-linked very long chain fatty acids, suggesting a defective sphingolipid alpha-hydroxylase encoded by the FAH1 gene. A yeast strain that lacks the FAH1 gene was resistant to syringomycin E, and failed to complement BW7. These results demonstrate that BW7 carries a mutation in the FAH1 gene, and that the lack of alpha-hydroxylated very long chain fatty acids in yeast sphingolipids confers resistance to syringomycin E.

Effect of temperature on the formation and inactivation of syringomycin E pores in human red blood cells and bimolecular lipid membranes.

The effects of temperature on the formation and inactivation of syringomycin E (SRE) pores were investigated with human red blood cells (RBCs) and lipid bilayer membranes (BLMs). SRE enhanced the RBC membrane permeability of 86Rb and monomeric hemoglobin in a temperature dependent manner. The kinetics of 86Rb and hemoglobin effluxes were measured at different temperatures and pore formation was found to be only slightly affected, while inactivation was strongly influenced by temperature. At 37 degrees C, SRE pore inactivation began 15 min after and at 20 degrees C, 40 min after SRE addition. At 6 degrees C, below the phase transition temperature of the major lipid components of the RBC membrane, no inactivation occurred for as long as 90 min. With BLMs, SRE induced a large current that remained stable at 14 degrees C, but at 23 degrees C it decreased over time while the single channel conductance and dwell time did not change. The results show that the temperature dependent inactivation of SRE pores is due to a decrease in the number of open pores.

Syringomycin E inhibition of Saccharomyces cerevisiae: requirement for biosynthesis of sphingolipids with very-long-chain fatty acids and mannose- and phosphoinositol-containing head groups.

Syringomycin E is an antifungal cyclic lipodepsinonapeptide that inhibits the growth of Saccharomyces cerevisiae by interaction with the plasma membrane. A screen conducted to find the yeast genes necessary for its fungicidal action identified two novel syringomycin E response genes, SYR3 and SYR4. A syr3 mutant allele was complemented by ELO2 and ELO3. These genes encode enzymes that catalyze the elongation of sphingolipid very long chain fatty acids. Tetrad analysis showed that SYR3 was ELO2. Strains with deletions of SYR3/ELO2 and ELO3 were resistant to syringomycin E, and lipid analyses of both mutants revealed shortened fatty acid chains and lower levels of sphingolipids. SYR4 was identified by Tn5 inactivation of genomic library plasmids that complemented a syr4 mutant allele. SYR4 was found to be identical to IPT1, which encodes the terminal sphingolipid biosynthetic enzyme, mannosyl-diinositolphosphoryl-ceramide synthase. Deletion Deltasyr4/ipt1 strains were viable, were resistant to syringomycin E, did not produce mannosyl-diinositolphosphoryl-ceramide, and accumulated mannosyl-inositolphosphoryl-ceramide. Accumulation of mannosyl-inositolphosphoryl-ceramide was not responsible for resistance since a temperature-sensitive secretory pathway mutant (sec14-3(ts)) accumulated this sphingolipid and was sensitive to syringomycin E. Finally, Deltacsg1/sur1 and Deltacsg2 strains defective in the transfer of mannose to inositolphosphoryl-ceramide were resistant to syringomycin E. These findings show that syringomycin E growth inhibition of yeast is promoted by the production of sphingolipids with fully elongated fatty acid chains and the mannosyl and terminal phosphorylinositol moieties of the polar head group.

Analysis of phosphoinositides in protein trafficking.

Phosphoinositides are key regulators of vesicle-mediated protein trafficking. Their roles include recruiting vesicle coat and effector proteins to the site of budding and promoting vesicle fusion. The intracellular levels of phosphoinositides and their localization to intracellular membranes are critical to their functions. An analytical procedure was developed that optimizes the recovery of radiolabeled cellular phosphoinositides. Quantitative analyses of yeast cellular phosphoinositides indicated that this approach is useful for examining the intracellular membrane phosphoinositide compositions related to trafficking phenomena. The approach will also enable investigators to determine whole-plant phosphoinositide compositions that have been difficult to achieve in the past. These analytical advances should be generally applicable to studies of phosphoinositide dynamics related to membrane trafficking in yeast, plant, and animal cells.

Direct involvement of phosphatidylinositol 4-phosphate in secretion in the yeast Saccharomyces cerevisiae.

The SEC14 gene encodes an essential phosphatidylinositol (PtdIns) transfer protein required for formation of Golgi-derived secretory vesicles in yeast. Suppressor mutations that rescue temperature-sensitive sec14 mutants provide an approach for determining the role of Sec14p in secretion. One suppressor, sac1-22, causes accumulation of PtdIns(4)P. SAC1 encodes a phosphatase that can hydrolyze PtdIns(4)P and certain other phosphoinositides. These findings suggest that PtdIns(4)P is limiting in sec14 cells and that elevation of PtdIns(4)P production can suppress the secretory defect. Correspondingly, we found that PtdIns(4)P levels were decreased significantly in sec14-3 mutants shifted to 37 degrees C and that sec14-3 cells could grow at an otherwise nonpermissive temperature (34 degrees C) when carrying a plasmid overexpressing PIK1, encoding one of two essential PtdIns 4-kinases. This effect is specific because overexpression of the other PtdIns 4-kinase gene (STT4) or a PtdIns 3-kinase gene (VPS34) did not rescue sec14-3 cells. To further address Pik1p function in secretion, two different pik1(ts) mutants were examined. Upon shift to restrictive temperature (37 degrees C), the PtdIns(4)P levels dropped by about 60% in both pik1(ts) strains within 1 h. During the same period, cells displayed a reduction (40-50%) in release of a secreted enzyme (invertase). However, similar treatment did not effect maturation of a vacuolar enzyme (carboxypeptidase Y). These findings indicate that, first, PtdIns(4)P limitation is a major contributing factor to the secretory defect in sec14 cells; second, Sec14p function is coupled to the action of Pik1p, and; third, PtdIns(4)P has an important role in the Golgi-to-plasma membrane stage of secretion.

SEC14-dependent secretion in Saccharomyces cerevisiae. Nondependence on sphingolipid synthesis-coupled diacylglycerol production.

The SEC14 gene in Saccharomyces cerevisiae encodes a phosphatidylinositol transfer protein required for secretory protein movement from the Golgi. Mutation of SAC1, a gene of unknown function, restores secretory flow in sec14-1(ts) strains. The existing model for the bypass of the sec14-1(ts) defect by sac1-22 involves stimulation of sphingolipid biosynthesis and, in particular, the synthesis of mannosyl-diinositolphosphoryl-ceramide with concomitant increases in Golgi diacylglycerol levels. To test this model, we disrupted IPT1, the mannosyl-diinositolphosphoryl-ceramide synthase of S. cerevisiae. Disruption of the IPT1 gene had no effect on the ability of sac1-22 to bypass sec14-1(ts). Furthermore, sphingolipid analysis of sec14-1(ts) and sec14-1(ts) sac1-22 strains showed that mannosyl-diinositolphosphoryl-ceramide synthesis was not stimulated in the bypass mutant. However, the sec14-1(ts) strain had elevated mannosyl-monoinositolphosphoryl-ceramide levels, and the sec14-1(ts) sac1-22 strain showed an 8-fold increase in phosphatidylinositol 4-phosphate along with a decrease in phosphatidylinositol 4,5-bisphosphate. Cellular diacylglycerol levels, measured by [14C]acetate incorporation, did not differ between the sec14-1(ts) and the sec14-1 sac1-22 bypass strains, although disruption of IPT1 in the bypass strain resulted in reduced levels. These data indicate that phosphatidylinositol 4-phosphate, rather than mannosyl-diinositolphosphoryl-ceramide, accumulates in the sec14-1(ts) sac1-22 bypass strain, and that Golgi diacylglycerol accumulation is not required for bypass of the sec14-1(ts) growth and secretory phenotypes.

Role of ergosterol in growth inhibition of Saccharomyces cerevisiae by syringomycin E.

The antifungal activity of the lipodepsipeptide syringomycin E from Pseudomonas syringae pv. syringae is modulated by sterols. To study the requirement of the predominant fungal sterol, ergosterol, in syringomycin E action, the sterol composition of Saccharomyces cerevisiae sterol auxotroph strain FY-14 was modified and sensitivity to syringomycin E examined. Cells containing solely ergosterol, cholesterol, beta-sitosterol or stigmasterol were sensitive to syringomycin E with the latter two being the most sensitive. Cells containing growth-promoting cholesterol were the most sensitive and those with growth-promoting ergosterol the least sensitive. It is concluded that sensitivity to syringomycin E is modulated by growth-promoting sterols and does not necessarily require ergosterol.

Efficacy of syringomycin E in a murine model of vaginal candidiasis.

Syringomycin E (SR-E), a new antifungal produced by the bacterium Pseudomonas syringae pv. syringae, was evaluated in a murine vaginal candidiasis model. In one study, mice were treated intravaginally b.i.d. for 4 days with drug carrier, SR-E 2% in either PEG-400 or PEG-ointment, or 1% clotrimazole as a positive control. Quantitative vaginal cultures were taken prior to treatment on day 1 and on days 5, 6, and 7. Both formulations showed a reduction of yeast colonization in the vaginas on day 5 (P< or =0.06 and P< or =0.03 for SR-E/PEG-400 and SR-E/PEG ointment, respectively) and SR-E/PEG ointment reduced the colonization on day 7 (P< or =0.06) when compared to carrier treated controls. In a second study, SR-E was formulated in Aquaphor at three higher concentrations of SR-E [3%, 6%, or 12% (w/v)]. SR-E showed dose-dependent efficacy. The 3% dose showed no effect while the 6% and 12% doses reduced the number of yeasts. The 12% dose showed a significant reduction on days 5 (P< or =0.01), 6 (P< or =0.06), and 7 (P< or =0.03) when compared with the drug carrier controls and on day 5 was more effective than clotrimazole (P< or =0.03). Clotrimazole did not significantly reduce the yeasts in the vagina until days 6 (P< or =0.01) and 7 (P< or =0.01) when compared to the drug carrier controls. No vaginal inflammatory response was evident by histological examination in uninfected animals treated with SR-E. No SR-E could be detected in plasma, kidney, or liver. SR-E (12%) was an effective treatment when compared to 1% clotrimazole.

Membrane sterol composition modulates the pore forming activity of syringomycin E in human red blood cells.

The effect of lipopeptide antifungal agent, syringomycin E (SRE) on the membrane permeability of human red blood cells (RBCs) was studied. SRE added to RBCs above a concentration of 2x106 molecules/cell (50 microgram/ml RBCs) caused a rapid and concentration dependent lysis of a small subpopulation of RBCs; the extent of this lysis remained unchanged as long as 100 min. During this time period the membranes of the unlysed cells had enhanced permeability for ions which was monitored by direct measurement of 86Rb flux. Both the extent of cell lysis and ion transport rate showed linear relationships with SRE concentration demonstrating a random distribution of SRE molecules in red blood cells. The kinetics of the 86Rb efflux suggested pore formation by syringomycin E. The pores had discrete life times and were eventually inactivated. The pores were also a pathway for efflux of monomeric haemoglobin. Alteration of the membrane sterol composition, i.e. depletion of cholesterol by 50% or partial ergosterol substitution of the cholesterol increased the SRE induced membrane permeability for 86Rb by two orders compared to membranes with unaltered sterol composition. This modification of the sterol composition promotes the pore forming activity of this lipopeptide in the membrane.

Cluster organization of ion channels formed by the antibiotic syringomycin E in bilayer lipid membranes.

The cyclic lipodepsipeptide, syringomycin E, when incorporated into planar lipid bilayer membranes, forms two types of channels (small and large) that are different in conductance by a factor of sixfold. To discriminate between a cluster organization-type channel structure and other possible different structures for the two channel types, their ionic selectivity and pore size were determined. Pore size was assessed using water-soluble polymers. Ion selectivity was found to be essentially the same for both the small and large channels. Their reversal (zero current) potentials with the sign corresponding to anionic selectivity did not differ by more than 3 mV at a twofold electrolyte gradient across the bilayer. Reduction in the single-channel conductance induced by poly(ethylene glycol)s of different molecular weights demonstrated that the aqueous pore sizes of the small and large channels did not differ by more than 2% and were close to 1 nm. Based on their virtually identical selectivity and size, we conclude that large syringomycin E channels are clusters of small ones exhibiting synchronous opening and closing.

Syringomycin action gene SYR2 is essential for sphingolipid 4-hydroxylation in Saccharomyces cerevisiae.

The Saccharomyces cerevisiae gene SYR2, necessary for growth inhibition by the cyclic lipodepsipeptide syringomycin E, is shown to be required for 4-hydroxylation of long chain bases in sphingolipid biosynthesis. Four lines of support for this conclusion are presented: (a) the predicted Syr2p shows sequence similarity to diiron-binding membrane enzymes involved in oxygen-dependent modifications of hydrocarbon substrates, (b) yeast strains carrying a disrupted SYR2 allele produced sphingoid long chain bases lacking the 4-hydroxyl group present in wild type strains, (c) 4-hydroxylase activity was increased in microsomes prepared from a SYR2 overexpression strain, and (d) the syringomycin E resistance phenotype of a syr2 mutant strain was suppressed when grown under conditions in which exogenous 4-hydroxysphingoid long chain bases were incorporated into sphingolipids. The syr2 strain produced wild type levels of sphingolipids, substantial levels of hydroxylated very long chain fatty acids, and the full complement of normal yeast sphingolipid head groups. These results show that the SYR2 gene is required for the 4-hydroxylation reaction of sphingolipid long chain bases, that this hydroxylation is not essential for growth, and that the 4-hydroxyl group of sphingolipids is necessary for syringomycin E action on yeast.

Properties of ionic channels formed by the antibiotic syringomycin E in lipid bilayers: dependence on the electrolyte concentration in the bathing solution.

Using the planar lipid bilayer technique, organization of ionic channels formed by the lipodepsipeptide antibiotic syringomycin E applied to one (cis) side of a lipid bilayer was studied. Low concentrations of NaCl (0.01-0.1 M) induced the opening and closing of two types of channels - "small" and "large". The large channels had single channel conductances approximately six times greater than those of the small channels. An increase in the NaCl concentration (0.6-1.0 M) decreased almost completely the chance to reveal the large channels. Although the syringomycin channels exhibited the anion selectivity within the entire range of NaCl concentrations in the bathing solutions (from 0.001 to 1.0 M) whereas the concentration gradients across the bilayers were 2 and 4, the transfer numbers for Cl-decreased with an increase in the mean NaCl concentration (from 0.83 for 0.005 M to 0.70 for 0.5 M). Moreover, at each mean value of NaCl concentration, all conductance levels had the same ion selectivity (identical reversal potential). These results suggest that at low NaCl concentrations the large channels are clusters of small channels which synchronously open and close, while at high electrolyte concentrations the screening of the charged groups responsible for channel interactions prevents the cluster formation. A new theoretical approach for the estimation of the channel radius and the number of elementary charges located at its inner surface (based on the experimental curve of the dependence of transfer number on the NaCl concentration) was developed. Based on this theoretical approach, the channel radius equal to 1 nm and one elementary charge located at its inner surface were obtained.

PCR Detection of Cyclic Lipodepsinonapeptide-Producing Pseudomonas syringae pv. syringae and Similarity of Strains.

Many strains of Pseudomonas syringae pv. syringae produce one of four classes of small cyclic lipodepsinonapeptides: syringomycins, syringostatins, syringotoxins, or pseudomycins. These metabolites are phytotoxic and growth inhibitory against a broad spectrum of fungi. Their production is dependent upon the expression of conserved biosynthesis and export genes syrB and syrD, respectively. PCR and oligonucleotide primers specific for a 752-bp fragment of syrB were used to identify cyclic lipodepsinonapeptide-producing strains of P. syringae pv. syringae. In contrast, PCR amplification with primers based on syrD did not always correlate with possession of the syrD gene, as indicated by Southern blot analysis, or with cyclic lipodepsinonapeptide production. Sequence comparisons of 400 nucleotides from the syrB PCR-amplified fragments showed 94% plot similarity among 27 strains. In a sequence phenogram, syringostatin and syringotoxin producers were grouped apart from syringomycin-producing strain B301D, with sequences that differed by eight and nine conserved base substitutions, respectively. PCR amplification of the 752-bp syrB fragment offers rapid and accurate detection of cyclic lipodepsinonapeptide-producing strains, and its sequence provides some predictive capabilities for identifying syringotoxin and syringostatin producers.