Membrane-permeabilizing Activity Research Articles

Identification of a unique nsaid, fluoro-loxoprofen with gastroprotective activity

We previously proposed that direct cytotoxicity of NSAIDs due to their membrane permeabilization activity, together with their ability to decrease gastric prostaglandin E2, contributes to production of gastric lesions. Compared to loxoprofen (LOX), fluoro-loxoprofen (F-LOX) has much lower membrane permeabilization and gastric ulcerogenic activities but similar anti-inflammatory activity. In this study, we examined the mechanism for this low ulcerogenic activity in rats. Compared to LOX, the level of gastric mucosal cell death was lower following administration of F-LOX. However, the gastric level of prostaglandin E2 was similar in response to treatment with the two NSAIDs. Oral pre-administration of F-LOX conferred protection against the formation of gastric lesions produced by subsequent administration of LOX and orally administered F-LOX resulted in a higher gastric pH value and mucus content. In the presence of a stimulant of gastric acid secretion, the difference in the ulcerogenic activity of F-LOX and LOX was less apparent. Furthermore, an increase in the mucus was observed in gastric cells cultured in the presence of F-LOX in a manner dependent of increase in the cellular level of cAMP. These results suggest that low ulcerogenic activity of F-LOX involves its both low direct cytotoxicity and protective effect against the development of gastric lesions. This protective effect seems to be mediated through an increase in a protective factor (mucus) and a decrease in an aggressive factor (acid).

SPP-3, a saposin-like protein of Caenorhabditis elegans, displays antimicrobial and pore-forming activity and is located in the intestine and in one head neuron

Caenopores belong to the saposin-like protein superfamily in Caenorhabditis elegans with 33 putative antimicrobial and pore-forming proteins. In this study, we analysed one selected member of this multifarious protein family, namely SPP-3, in detail, as its coding gene has been described to be inducible after bacterial challenge. The recombinant protein was antimicrobially active against a wide range of gram-negative and gram-positive bacteria and displayed membrane-permeabilizing and pH-dependent pore-forming activity. Promoter activity of the respective gene, spp-3, was localized to the intestine and the head neuron SDQR. While gene silencing had no apparent effect on the number of surviving Escherichia coli bacteria in the intestine, it increased the egg laying significantly. Accordingly, SPP-3 is a protein with antimicrobial activity that is presumably part of the redundant armamentarium of effector proteins in the worm’s intestine, may help to protect neurons, and appears to be involved in regulating reproduction.

Development of a small anti‐cancer molecule targeting both the intrinsic and extrinsic pathways of apoptosis

Our chemical genetic screen for small molecules that increase lifespan in yeast cellular models of aging by targeting mitochondria‐controlled apoptosis identified lithocholic acid (LCA) as one of such molecules. Aging is one of the major risk factors of cancer, an age‐related disease whose onset can be delayed and incidence reduced by certain pharmacological antiaging interventions. We therefore sought to examine if LCA exhibits an anti‐tumor effect in cultured human cancer cells by activating certain anti‐cancer processes that may play an essential role in cellular aging. We found that LCA, at concentrations that are not cytotoxic to primary cultures of human neurons, kills the neuroblastoma (NB) cell lines BE(2)‐m17, SK‐n‐SH, SK‐n‐ MCIXC and Lan‐1. In the case of BE(2)‐m17, SK‐n‐SH and SK‐n‐ MCIXC cells, the LCA anti‐tumor effect is due to apoptotic cell death. In contrast, the LCA‐triggered death of Lan‐1 cells is not caused by apoptosis. Our findings imply that LCA kills BE(2)‐m17 and SK‐n‐MCIXC cell lines by triggering not only the intrinsic (mitochondrial) apoptotic cell death pathway driven by mitochondrial outer membrane permeabilization and initiator caspase‐9 activation, but also the extrinsic (death receptor) pathway of apoptosis involving activation of the initiator caspase‐ 8. Our data suggest a molecular mechanism underlying a potent and selective anti‐tumor effect of LCA in cultured human NB cells. Importantly, we found that a similar mechanism underlies a broad anti‐tumor effect of LCA in cultured mammalian cancer cells derived from different tissues and organisms. Supported by NSERC of Canada.

Electrical Aspects of Membrane Permeabilization by New Polycationic Peptides Derived from the Cry11Bb Protoxin

The permeabilization of mitochondrial and plasma membranes by synthetic polycationic peptides derived from the Cry11Bb protoxin was studied. The peptides were designed with the aim to further study of their antimicrobial and anticancer activities. It was observed that the membrane permeabilizing activity of these polycationic peptides strongly increased by the transmembrane potential (minus inside). This phenomenon was confirmed by the study of the artificial planar membrane permeabilization: applying 50 mV to the planar membrane (minus to the trans side) during 30 sec induced time-dependent increase in the transmembrane current in the presence of a peptide added to the cis side, while subsequent application of the opposite potential caused its decrease. We also observed that the activation of the cell suicide mechanism, which partially revealed in phosphatidylserine exposure at the cell surface, significantly increased the plasma membrane permeabilization by polycationic peptides. Some peptide topology characteristics, such as the value and the orientation of the electrical dipole moment(s) interacting with the membrane dipole potential seem to also be important factors influencing the membrane-permeabilizing activity of polycationic peptides. In general, our data are consistent with the concept that various electrical properties of biomembranes (the transmembrane potential, membrane dipole potential and the surface charge) might explain at least partially certain selectivity of antimicrobial and anticancer activities of many natural and synthetic polycationic peptides. (Financial support: Colciencias (Colombia) research grant #111840820380).

Antimicrobial Peptides: Correlation Between Pore Forming Potency in Lipid Vesicles and Antimicrobial, Hemolytic and Cytotoxic Activity

The rapid rise in drug-resistant pathogenic microorganisms has generated elevated interest in antimicrobial peptides (AMPs), which have been proposed to act largely based on the potential to induce target cell membrane permeabilization. To search for novel AMPs, we designed a 26-residue combinatorial peptide library which was screened for potent membrane pore-formers using a lipid-vesicle-based leakage assay. Selected pore-forming peptides also showed good antimicrobial activity against yeast, gram-positive and gram-negative bacteria. To determine if peptides with higher activity could be identified with the same principle, we used one of the original selected peptides as a template, to design a combinatorial peptide library iteration. This iterative library was screened using a recently modified orthogonal fluorescence-based liposome assay that specifically selects for peptides that can induce highly potent, long-lived pores. The peptides in the iterative library, which had an average pore forming activity that was significantly higher than the ones in the original library, was consequently screened at a much higher stringency to find the most potent members. We identified a conserved family of sequences with extremely high membrane permeabilizing activity against lipid vesicles. These selected positives from the iteration library screen were synthesized and tested in antimicrobial and hemolytic assays. Despite their much higher vesicle permeabilization activity, their antimicrobial activities are only slightly better than the positives identified in the original library. These results support the idea that antimicrobial activity is a complex bioactivity with important factors that extend beyond the simple concept of membrane permeabilization.

Antibacterial and Antifungal Activities of New Acylated Derivatives of Epigallocatechin Gallate

(−)-Epigallocatechin-3-O-gallate (EGCG) has useful antiviral, antimicrobial, antitoxin, and antitumor properties. Previously, Mori et al. (2008) found that addition of long acyl chains (C16–18) to EGCG enhanced its anti-influenza virus activity up to 44-fold. The chemical stability of EGCG against oxidative degradation was also enhanced by acylation. We further evaluated the in vitro activity spectrum of the EGCG derivatives against a wide range of bacteria and fungi. A series of EGCG O-acyl derivatives were synthesized by lipase-catalyzed transesterification. These derivatives exhibited several-fold higher activities than EGCG, particularly against Gram-positive organisms. Antifungal MICs of the derivatives were also two to fourfold lower than those of EGCG. The activities of the EGCG derivatives against Gram-negative bacteria were not distinguishable from those of EGCG. Among the derivatives evaluated, MICs of dioctanoate and palmitate (C16) for 17 Staphylococcus aureus strains were 4–32 μg/ml, although MIC of EGCG for these 17 strains was ≥128 μg/ml. C16 demonstrated rapid bactericidal activity against methicillin-resistant S. aureus (MRSA) ATCC43300 at ≥16 μg/ml. The enhanced activity of C16 against S. aureus was supported by its increased membrane-permeabilizing activity determined by increased SYTOX Green uptake. The EGCG derivatives were exported in Escherichia coli using the efflux pump AcrAB–TolC. The tolC deletion mutant exhibited higher sensitivity to EGCG and the derivatives than wild-type. Addition of long alkyl chains to EGCG significantly enhanced its activities against several bacteria and fungi, particularly against S. aureus including MRSA. C16 might potentially become under specified circumstances an alternative or supplement to antibiotics and disinfectants in the future.

Lithocholic bile acid selectively kills neuroblastoma cells, while sparing normal neuronal cells

Aging is one of the major risk factors of cancer. The onset of cancer can be postponed by pharmacological and dietary anti-aging interventions. We recently found in yeast cellular models of aging that lithocholic acid (LCA) extends longevity. Here we show that, at concentrations that are not cytotoxic to primary cultures of human neurons, LCA kills the neuroblastoma (NB) cell lines BE(2)-m17, SK-n-SH, SK-n-MCIXC and Lan-1. In BE(2)-m17, SK-n-SH and SK-n-MCIXC cells, the LCA anti-tumor effect is due to apoptotic cell death. In contrast, the LCA-triggered death of Lan-1 cells is not caused by apoptosis. While low concentrations of LCA sensitize BE(2)-m17 and SK-n-MCIXC cells to hydrogen peroxide-induced apoptotic cell death controlled by mitochondria, these LCA concentrations make primary cultures of human neurons resistant to such a form of cell death. LCA kills BE(2)-m17 and SK-n-MCIXC cell lines by triggering not only the intrinsic (mitochondrial) apoptotic cell death pathway driven by mitochondrial outer membrane permeabilization and initiator caspase-9 activation, but also the extrinsic (death receptor) pathway of apoptosis involving activation of the initiator caspase-8. Based on these data, we propose a mechanism underlying a potent and selective anti-tumor effect of LCA in cultured human NB cells. Moreover, our finding that LCA kills cultured human breast cancer and rat glioma cells implies that it has a broad anti-tumor effect on cancer cells derived from different tissues and organisms.

Permeabilization of mitochondria and red blood cells by polycationic peptides BTM-P1 and retro-BTM-P1

Mitochondrial and plasma membrane permeabilization by polycationic peptides BTM-P1 and retro-BTM-P1 were studied. BTM-P1 was more active than its retro-analog. In the sucrose medium, the capacity of BTM-P1 to permeabilize mitochondria was lower than in salt media. In contrast, retro-BTM-P1 showed the lowest activity in the KCl medium. The efficacy of both peptides to permeabilize red blood cells was higher in the sucrose medium and depended on the nature of salt in high ionic strength media. BTM-P1, but not retro-BTM-P1, induced biphasic change in light dispersion of red blood cells with artificially generated high transmembrane potential: the initial phase of fast cell shrinkage preceded the subsequent phase of cell swelling. The shrunken red blood cells demonstrated increased sensitivity to BTM-P1 that might be explained by the cell suicide mechanism via phosphatidylserine exposure at the cell surface. As a working hypothesis, we assume that some peptide topology characteristics, such as the orientation and values of the total and local electrical dipole moments, interacting with the membrane dipole potential, as well as the asymmetric distribution of polar and non-polar side chains are important factors affecting the membrane-permeabilizing activity of polycationic peptides.

Azacitidine-resistant SKM1 myeloid cells are defective for AZA-induced mitochondrial apoptosis and autophagy

Azacitidine (AZA) is the current treatment for patients with high-risk myelodysplastic syndrome, but resistance is a common feature of AZA-treated patients. To investigate the mechanisms associated with AZA resistance in vitro, we generated AZA-resistant SKM1 myeloid cells, called hereafter AZA-R. AZA-R cells exhibit impaired mitochondrial membrane permeabilization and caspase activation in response to AZA compared to their AZA-sensitive (AZA-S) counterpart. AZA induced LC3-II accumulation and cathepsin B activity in AZA-S cells, two hallmarks of autophagy. AZA-R cells displayed increased basal autophagy but are resistant to AZA-mediated autophagy. Inhibition of autophagy using LC3 siRNA revealed that autophagy is protective in AZA-S cells and AZA-R cells in basal conditions. By contrast, AZA-R cells exhibited impaired autophagy in response to AZA. Collectively, our findings indicate that AZA promotes apoptosis and autophagy in SKM1 cells, and that AZA-R cells are resistant to both apoptosis and autophagy induced by AZA.

Differential Sensitivities of Malignant and Normal Skin Cells to Nanosecond Pulsed Electric Fields

Pulsed electric fields with nanosecond duration and high amplitude have effects on biological subjects and bring new venue in disease diagnosis and therapy. To address this respect, we investigated the responses of paired tumor and normal human skin cells - a basal cell carcinoma (BCC) cell line, and its sister normal cell line (TE) - to nanosecond, megavolt-per-meter pulses. When BCC (TE 354.T) and TE (TE 353.SK) cells, cultured under standard conditions, were exposed to 30 ns, 3 MV/m, 50 Hz pulses and tested for membrane permeabilization, viability, morphology, and caspase activation, we found that nanoelectropulse exposure: 1) increased cell membrane permeability in both cell lines but to a greater extent in BCC cells than in normal cells; 2) decreased cell viabilities with BCC cells affected more than normal cells; 3) induced morphological changes in both cell lines including condensed and fragmented chromatin with enlarged nuclei; 4) induced about twice as much caspase activation in BCC cells compared to normal cells. We concluded that in paired tumor and normal skin cell lines, the response of the tumor cells to nanoelectropulse exposure is stronger than the response of normal cells, indicating the potential for selectivity in therapeutic applications.

Membrane-damaging activity of Taiwan cobra cardiotoxin 3 is responsible for its bactericidal activity

This study investigates the causal relationship between membrane-damaging activity and bactericidal activity of Naja naja atra (Taiwan cobra) cardiotoxin 3 (CTX3). CTX3 showed greater inhibitory activity for the growth of Staphylococcus aureus (Gram-positive bacteria) relative to that of Escherichia coli (Gram-negative bacteria). The CTX3 antibacterial activity is positively correlated with the increase in membrane permeability of bacterial cells. Morphological examination showed that CTX3 disrupted bacterial membrane integrity.CTX3 showed similar binding capability with lipopolysaccharide (LPS) and lipoteichoic acid (LTA), and destabilization of LPS layer and inhibition of LTA biosynthesis on cell wall increased the CTX3 bactericidal effect on E. coli. and S. aureus, respectively. Compared with that of E. coli, CTX3 notably permeabilized model membrane of S. aureus. CTX3 membrane-damaging activity was inhibited by LPS and LTA, while increasing the CTX3 concentration counteracted the inhibitory action of LPS and LTA. Oxidation of Met residues on loop II of CTX3 simultaneously reduced the membrane-permeabilizing activity and bactericidal effect of CTX3. Taken together, our data indicate that CTX3 bactericidal activity depends highly on its ability to induce membrane permeability.

Synthesis and biological evaluation of loxoprofen derivatives

Non-steroidal anti-inflammatory drugs (NSAIDs) achieve their anti-inflammatory actions through an inhibitory effect on cyclooxygenase (COX). Two COX subtypes, COX-1 and COX-2, are responsible for the majority of COX activity at the gastrointestinal mucosa and in tissues with inflammation, respectively. We previously suggested that both gastric mucosal cell death due to the membrane permeabilization activity of NSAIDs and COX-inhibition at the gastric mucosa are involved in NSAID-induced gastric lesions. We have also reported that loxoprofen has the lowest membrane permeabilization activity among the NSAIDs we tested. In this study, we synthesized a series of loxoprofen derivatives and examined their membrane permeabilization activities and inhibitory effects on COX-1 and COX-2. Among these derivatives, 2-{4′-hydroxy-5-[(2-oxocyclopentyl)methyl]biphenyl-2-yl}propanoate 31 has a specificity for COX-2 over COX-1. Compared to loxoprofen, oral administration of 31 to rats produced fewer gastric lesions but showed an equivalent anti-inflammatory effect. These results suggest that 31 is likely to be a therapeutically beneficial and safer NSAID.

Amphipathic Tail-anchoring Peptide and Bcl-2 Homology Domain-3 (BH3) Peptides from Bcl-2 Family Proteins Induce Apoptosis through Different Mechanisms

Bcl-2 homology domain-3 (BH3) peptides are potent cancer therapeutic reagents that target regulators of apoptotic cell death in cancer cells. However, their cytotoxic effects are affected by different expression levels of Bcl-2 family proteins. We recently found that the amphipathic tail-anchoring peptide (ATAP) from Bfl-1, a bifunctional Bcl-2 family member, produced strong pro-apoptotic activity by permeabilizing the mitochondrial outer membrane. Here, we test whether the activity of ATAP requires other cellular factors and whether ATAP has an advantage over the BH3 peptides in targeting cancer cells. Confocal microscopic imaging illustrates specific targeting of ATAP to mitochondria, whereas BH3 peptides show diffuse patterns of cytosolic distribution. Although the pro-apoptotic activities of BH3 peptides are largely inhibited by either overexpression of anti-apoptotic Bcl-2 or Bcl-xL or nullification of pro-apoptotic Bax and Bak in cells, the pro-apoptotic function of ATAP is not affected by these cellular factors. Reconstitution of synthetic ATAP into liposomal membranes results in release of fluorescent molecules of the size of cytochrome c from the liposomes, suggesting that the membrane permeabilizing activity of ATAP does not require additional protein factors. Because ATAP can target to the mitochondrial membrane and its pro-apoptotic activity does not depend on the content of Bcl-2 family proteins, it represents a promising candidate for anti-cancer drugs that can potentially overcome the intrinsic apoptosis-resistant nature of cancer cells.

Deciphering a mechanism of membrane permeabilization by α-hordothionin peptide

α-Hordothionin (αHTH) belongs to thionins, the plant antimicrobial peptides with membrane-permeabilizing activity which is associated with broad-range antimicrobial activity. Experimental data have revealed a phospholipid-binding site and indicated formation of ion channels as well as membrane disruption activity of thionin. However, the mechanism of membrane permeabilization by thionin remained unknown. Here it is shown that thionin is a small water-selective channel. Unbiased high-precision molecular modeling revealed formation of a water-selective pore running through the αHTH double α-helix core when the peptide interacted with anions. Anion-induced unfolding of the C-end of the α2-helix opened a pore mouth. The pore started at the α2 C-end between the hydrophilic and the hydrophobic regions of the peptide surface and ended in the middle of the unique hydrophobic region at the C-end of the α1-helix. Highly conserved residues including cysteines and tyrosine lined the pore walls. A large positive electrostatic potential accumulated inside the pore. The narrow pore was, nonetheless, sufficient to accommodate at least one water molecule along the channel except for two constriction sites. Both constriction sites were formed by residues participating in the phospholipid-binding site. The channel properties resembled that of aquaporins with two selectivity filters, one at the entrance, inside the α2 C-end cavity, and a second in the middle of the channel. It is proposed that the αHTH water channel delivers water molecules to the bilayer center that leads to local membrane disruption. The proposed mechanism of membrane permeabilization by thionins explains seemingly controversial experimental data.

Biophysical Characterization of Peptide Membrane Interactions and Membrane Permeabilization

Certain membrane-active amphipathic peptides, such as antimicrobial peptides, possess a characteristic feature of having several cationic amino acid residues at either N- or C-terminus. In an attempt to determine the functional significance of C-terminal cationic residues, we have studied the membrane binding mode and membrane pore formation by a 20-amino acid synthetic amphipathic peptide. Two cationic residues were replaced to Leu or Glu to see how the membrane binding and permeabilization activities of the peptide are modulated by charge neutralization and charge reversal mutations. The depth of membrane insertion of the wild type and mutant peptides was determined by measuring the quenching of the fluorescence of a single, C-terminally located tryptophan by dibromo-phosphatidylcholines brominated at various positions of the acyl chains, using pure POPC and 70% POPC + 30% POPG in lipid vesicles. In case of pure POPC membranes, Trp in all three peptides was located at around 9 Å from the membrane center, while for 30% POPG, Trp was inserted much deeper. Studies on peptide-mediated calcein release from phospholipid vesicles were conducted to see if the peptides are able to cause membrane pore formation and content release. Peptides with positively charged residues showed maximum calcein release for both lipid compositions whereas negative or hydrophobic residues were less effective in vesicle permeabilization. Preliminary ATR-FTIR experiments using both POPC and POPG have been performed to identify the orientation of the peptides within the lipid bilayer and their effect of the lipid order.

Showing the Potential of the Lifetime-Based Dye Leakage Assay: All-Or-None Membrane Permeabilization by Fungicidal Lipopeptides from Bacillus Subtilis QST 713

The fungicidal activity of Bacillus subtilis QST713, based mainly on the production of cyclic lipopeptides of the fengycin (FEs), surfactin, and iturin families, has been utilized for the highly effective and environmentally safe protection of crops against a variety of pathogens. The mixed population of native FEs forms micelles which solubilize individual FEs such as agrastatin 1 (AS1) that are otherwise rather insoluble on their own but promote the membrane permeabilizing activity of the mixture. Fluorescence lifetime-based calcein (Softmatter, 2009, 5, 2849-51) efflux measurements, isothermal titration calorimetry and electron microscopy show that these FEs show a unique scenario of membrane permeabilization. Poor miscibility of FEs with lipid induces stable, long-lived pores in 10% of the vesicles at only ≈ 1uM free FE and in 15% of the vesicles at 10 uM. We explain why this all-or-none leakage accounts for the killing of virtually all fungi whereas the same extent of graded leakage would likely be biologically irrelevant. Then, crystallization of AS1 and micellization of plipastatins cause a cut-off in leakage at 15% that might regulate the biological activity of FEs, protecting Bacillus and plant membranes. The fact that FE micelles solubilize only about 10 mol-% fluid lipid resembles the behavior of detergent resistance.

Effect of proline position on the antimicrobial mechanism of buforin II

Buforin II (BF2) is a histone-derived antimicrobial peptide that causes cell death by translocating across membranes and interacting with nucleic acids. It contains one proline residue critical for its function. Previous research found that mutations replacing proline lead to decreased membrane translocation and antimicrobial activity as well as increased membrane permeabilization. This study further investigates the role of proline in BF2's antimicrobial mechanism by considering the effect of changing proline position on membrane translocation, membrane permeabilization, and antimicrobial activity. For this purpose, four mutants were made with proline substitution (P11A) or relocation (P11A/G7P, P11A/V12P, P11A/V15P). These mutations altered the amount of helical content. Although antimicrobial activity correlated with the α-helical content for the peptides containing proline, membrane translocation did not. This observation suggests that factors in BF2's bactericidal mechanism other than translocation must be altered by these mutations. To better explain these trends we also measured the nucleic acid binding and membrane permeabilization of the mutant peptides. A comparison of mutant and wild type BF2 activity revealed that BF2 relies principally on membrane translocation and nucleic acid binding for antimicrobial activity, although membrane permeabilization may play a secondary role for some BF2 variants. A better understanding of the role of proline in the BF2 antimicrobial mechanism will contribute to the further design and development of BF2 analogs. Moreover, since proline residues are prevalent among other antimicrobial peptides, this systematic characterization of BF2 provides general insights that can promote our understanding of other systems.

Properties and Synthesis of 2-{2-Fluoro (or Bromo)-4-[(2-oxocyclopentyl)methyl]phenyl}propanoic Acid: Nonsteroidal Anti-inflammatory Drugs with Low Membrane Permeabilizing and Gastric Lesion-Producing Activities

We previously proposed that membrane permeabilization activity of NSAIDs is involved in NSAID-induced gastric lesions. We here synthesized derivatives of loxoprofen that have lower membrane permeabilization activity than other NSAIDs. Compared to loxoprofen, the derivatives 10a and 10b have lower membrane permeabilization activity and their oral administration produced fewer gastric lesions but showed an equivalent anti-inflammatory effect. These results suggest that 10a and 10b are likely to be therapeutically beneficial as safer NSAIDs.

Comparative study of the membrane-permeabilizing activities of mastoparans and related histamine-releasing agents in bacteria, erythrocytes, and mast cells

The membrane-permeabilizing activities of mastoparans and related histamine-releasing agents were compared through measurements of K+ efflux from bacteria, erythrocytes, and mast cells. Changes in bacterial cell viability, hemolysis, and histamine release, as well as in the shape of erythrocytes were also investigated. The compounds tested were mastoparans (HR1, a mastoparan from Polistes jadwagae, and a mastoparan from Vespula lewisii), granuliberin R, mast cell-degranulating peptide, and compound 48/80, as well as antimicrobial peptides, such as magainin I, magainin II, gramicidin S, and melittin. We used a K+-selective electrode to determine changes in the permeability to K+ of the cytoplasmic membranes of cells. Consistent with the surface of mast cells becoming negatively charged during histamine release, due to the translocation of phosphatidylserine to the outer leaflet of the cytoplasmic membrane, histamine-releasing agents induced K+ efflux from mast cells, dependent on their ability to increase the permeability of bacterial cytoplasmic membranes rich in negatively charged phospholipids. The present results demonstrated that amphiphilic peptides, possessing both histamine-releasing and antimicrobial capabilities, induced the permeabilization of the cytoplasmic membranes of not only bacteria but mast cells. Mastoparans increased the permeability of membranes in human erythrocytes at higher concentrations, and changed the normal discoid shape to a crenated form. The structural requirement for making the crenated form was determined using compound 48/80 and its constituents (monomer, dimer, and trimer), changing systematically the number of cationic charges of the molecules.

Cell wall integrity and high osmolarity glycerol pathways are required for adaptation of Alternaria brassicicola to cell wall stress caused by brassicaceous indolic phytoalexins

Camalexin, the characteristic phytoalexin of Arabidopsis thaliana, inhibits growth of the fungal necrotroph Alternaria brassicicola. This plant metabolite probably exerts its antifungal toxicity by causing cell membrane damage. Here we observed that activation of a cellular response to this damage requires cell wall integrity (CWI) and the high osmolarity glycerol (HOG) pathways. Camalexin was found to activate both AbHog1 and AbSlt2 MAP kinases, and activation of the latter was abrogated in a AbHog1 deficient strain. Mutant strains lacking functional MAP kinases showed hypersensitivity to camalexin and brassinin, a structurally related phytoalexin produced by several cultivated Brassica species. Enhanced susceptibility to the membrane permeabilization activity of camalexin was observed for MAP kinase deficient mutants. These results suggest that the two signalling pathways have a pivotal role in regulating a cellular compensatory response to preserve cell integrity during exposure to camalexin. AbHog1 and AbSlt2 deficient mutants had reduced virulence on host plants that may, at least for the latter mutants, partially result from their inability to cope with defence metabolites such as indolic phytoalexins. This constitutes the first evidence that a phytoalexin activates fungal MAP kinases and that outputs of activated cascades contribute to protecting the fungus against antimicrobial plant metabolites.