Antibacterial peptides isolated from insects

Antimicrobial activity in the egg wax of the African cattle tick Amblyomma hebraeum (Acari: Ixodidae)

Eggs of the tick Amblyomma hebraeum Koch (Acari: Ixodidae) inhibited the growth of Escherichia coli and Serratia marcescens (Gram-negative bacteria) in solid culture, but not the growth of Staphylococcus epidermidis, and only marginally the growth of Bacillus subtilis (Gram-positive bacteria). When egg wax was extracted with chloroform/methanol (2:1), the extract contained antibacterial activity, but the denuded eggs did not. When assayed against bacteria in liquid culture, the aqueous phase inhibited the growth of S. epidermidis. However, the activity against E. coli was lost during extraction. The antimicrobial component of the aqueous phase was heat stable (100 degrees C for 10 min), resistant to proteinase K (15 min at 55 degrees C) and to pronase (30 min at 37 degrees C). The antibacterial activity in the aqueous phase increased the permeability of the cell membrane of susceptible bacterial cells within 30 min. However, lysis of the cells was detected by optical density measurements (OD(600 nm)) only after 1.5 h. The most evident cytological changes observed by transmission electron microscopy were a thickening of the cell wall and the appearance of numerous electron lucent areas within the cytoplasm of treated bacteria. Gené's organ, the egg-waxing organ in ticks, grew enormously during the first 16 days post-engorgement, and gained antimicrobial activity by day 10 (when oviposition began). This suggests that Gené's organ is the major source of the antibacterial substance in the egg wax. The vitellogenic hormone in A. hebraeum, 20-hydroxyecdysone, when injected into recently engorged females, did not stimulate growth of Gené's organ or precocious secretion of antimicrobial activity.

Antagonists of Hsp16.3, a low-molecular-weight mycobacterial chaperone and virulence factor, derived from phage-displayed peptide libraries

The persistence of Mycobacterium tuberculosis is a major cause of concern in tuberculosis (TB) therapy. In the persistent mode the pathogen can resist drug therapy, allowing the possibility of reactivation of the disease. Several protein factors have been identified that contribute to persistence, one of them being the 16-kDa low-molecular-weight mycobacterial heat shock protein Hsp16.3, a homologue of the mammalian eye lens protein alpha-crystallin. It is believed that Hsp16.3 plays a key role in the persistence phase by protecting essential proteins from being irreversibly denatured. Because of the close association of Hsp16.3 with persistence, an attempt has been made to develop inhibitors against it. Random peptide libraries displayed on bacteriophage M13 were screened for Hsp16.3 binding. Two phage clones were identified that bind to the Hsp16.3 protein. The corresponding synthetic peptides, an 11-mer and a 16-mer, were able to bind Hsp16.3 and inhibit its chaperone activity in vitro in a dose-dependent manner. Little or no effect of these peptides was observed on alphaB-crystallin, a homologous protein that is a key component of human eye lens, indicating that there is an element of specificity in the observed inhibition. Two histidine residues appear to be common to the selected peptides. Nuclear magnetic resonance studies performed with the 11-mer peptide indicate that in this case these two histidines may be the crucial binding determinants. The peptide inhibitors of Hsp16.3 thus obtained could serve as the basis for developing potent drugs against persistent TB.

Antimicrobial peptides (AMPs): peptide structure and mode of action

Antimicrobial peptides (AMPs) have been isolated and characterized from tissues and organisms representing virtually every kingdom and phylum. Their amino acid composition, amphipathicity, cationic charge, and size allow them to attach to and insert into membrane bilayers to form pores by 'barrel-stave', 'carpet' or 'toroidal-pore' mechanisms. Although these models are helpful for defining mechanisms of AMP activity, their relevance to resolving how peptides damage and kill microorganisms still needs to be clarified. Moreover, many AMPs employ sophisticated and dynamic mechanisms of action to carry out their likely roles in antimicrobial host defense. Recently, it has been speculated that transmembrane pore formation is not the only mechanism of microbial killing by AMPs. In fact, several observations suggest that translocated AMPs can alter cytoplasmic membrane septum formation, reduce cell-wall, nucleic acid, and protein synthesis, and inhibit enzymatic activity. In this review, we present the structures of several AMPs as well as models of how AMPs induce pore formation. AMPs have received special attention as a possible alternative way to combat antibiotic-resistant bacterial strains. It may be possible to design synthetic AMPs with enhanced activity for microbial cells, especially those with antibiotic resistance, as well as synergistic effects with conventional antibiotic agents that lack cytotoxic or hemolytic activity.

Convergent evolution of invertebrate defensins and nematode antibacterial factors

Antibacterial factors (ABFs) are secreted polypeptides that have an important role in the innate immune system of nematodes. Comparison of these polypeptides revealed similarity in bioactivity, protein sequence and 3D structure, suggesting that they originated from a common ancestor. Comparison of gene organization of nematode ABF genes revealed that all except one contain a Phase 0 intron at a conserved location. The intron phase and location are congruent with the postulated intron gain rules, suggesting a gain of intron before duplication and divergence of the ancestral gene. Although nematode ABFs display similarity in activity and structure to invertebrate (arthropod and mollusk) defensins, lack of sequence similarity and the different genomic organization suggest that these two polypeptide families exhibit convergent evolution.

Antimicrobial peptides as microbicidal contraceptives: prophecies for prophylactics--a mini review

The global increase in human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS) and sexually transmitted infections (STIs) has led to the introduction of barrier methods, such as condoms. However, drawbacks associated with condoms, such as men being reluctant to use them and women being unable to negotiate their use, have led to the search for better and acceptable alternatives, namely the microbicides. These are gel formulations that, when used prior to sexual intercourse, protect against the transmission of HIV and other STIs. However, after observing the side-effects of nonoxynol-9, a component of the microbicidal formulations available on the market, the focus has shifted to natural available compounds demonstrating the preferred protective effects. Antimicrobial peptides (AMPs) are one such group of compounds present in a wide range of organisms from bacteria to humans. The existing 750 or so, low-molecular-weight, cationic charged peptides are classified into five major groups based on their three-dimensional structure obtained by nuclear magnetic resonance studies. The hypothesized mode of action seems to be the interaction of the positively charged peptides with the negatively charged phospholipids present on the surface of the cell membrane. Various studies have demonstrated the effect of several AMPs, namely, defensins, protegrins, cathelicidins, cecropins, polyphemusins, magainins and melittins, against various STI-causing pathogens and HIV/herpes simplex virus, both in vitro and in vivo. The contraceptive efficacies of magainin and nisin in vitro and in vivo are worth mentioning. We believe these peptides are suitable candidates in the development of newer mechanism-based microbicides in future.

An insect antibacterial peptide-based drug delivery system

The ability of the short, proline-rich native antibacterial peptides to penetrate bacterial and host cells suggests the utility of these transport systems in delivering peptidic cargo into cells. We studied the uptake of pyrrhocoricin and its most potent dimeric analogue by bacteria as well as human dendritic cells and fibroblasts. Native pyrrhocoricin entered the susceptible organism Escherichia coli very efficiently and the nonsusceptible bacterium Staphylococcus aureus to a significant degree. The antibacterial peptide also penetrated human monocyte-derived dendritic cells. It failed, however, to enter fibroblasts, whereas the designer analogue Pip-pyrr-MeArg dimer penetrated all the cell types that were studied. When glucoincretin hormone Glp-1 fragment 7-36 was cosynthesized with the dimer, the antibacterial peptide derivative lost its ability to cross the bacterial membrane layer. In contrast, a chimera of the Pip-pyrr-MeArg dimer and two copies of a shorter (nine residues) class I major histocompatibility complex epitope successfully entered bacterial and mammalian cells. While the Pip-pyrr-MeArg dimer was not immunogenic when inoculated into mice, the chimera elicited a strong cytotoxic T-cell response, indicating the maintenance of the antigenic integrity of the cargo in the peptide conjugate. The chimera when tested for its immunological properties activated human dendritic cells significantly more strongly than any of the two independent fragments alone, yet lacked mammalian cell toxicity. These results confirm the utility of designed pyrrhocoricin analogues for delivery of peptidic cargo across cell membranes in general, and their potential as carriers for epitope-based vaccines in particular.

Biological evaluation of Tyr6 and Ser7 modified drosocin analogues

An array of analogues of the cationic antimicrobial peptide drosocin was synthesized containing substitutions of Tyr6 and Ser7 in order to increase the proteolytic stability. Stabilizing the N-terminus with unnatural amino acids increased the serum stability of analogues by almost a factor 30 over an 8 h period.

Arthropod defensins illuminate the divergence of scorpion neurotoxins

Defensins are phylogenetically ancient antibacterial polypeptides found in plants and animals. Isolation of the cDNA and genomic sequences encoding the scorpion (Leiurus quinquestriatus hebraeus) defensin revealed similarity to scorpion neurotoxins in gene organization (two exons and a phase I intron) and intron characteristics (conserved acceptor, donor and putative branch sites). This commonality, alongside a similar core structure, protein sequence and bioactivity suggest that arthropod defensins and scorpion neurotoxins share a common ancestor. Interestingly, phylogenetic analysis of defensins and scorpion neurotoxins illuminates for the first time a putative evolutionary trajectory for scorpion sodium and potassium channel neurotoxins.

Function and therapeutic potential of host defence peptides

Cationic host defence (antimicrobial) peptides are an important component of the innate immune systems of a wide variety of plants, animals, and bacteria. Although most of these compounds have direct antimicrobial activities under specific conditions, a greater appreciation for the diversity of functions of these molecules is beginning to develop in the field. In addition to their directly antimicrobial activities, they also have a broad spectrum of activity on the host immune system, with both pro-inflammatory and anti-inflammatory effects being invoked. Increasingly sophisticated approaches to understand the role of host defence peptides in modulating innate immunity are already serving to guide the development of novel therapeutics.

A new class (penaeidin class 4) of antimicrobial peptides from the Atlantic white shrimp (Litopenaeus setiferus) exhibits target specificity and an independent proline-rich-domain function

A highly pure, chemically defined representative of a new class of antimicrobial peptide from the Atlantic white shrimp (Litopenaeus setiferus), penaeidin class 4 [Pen4-1 (penaeidin class 4 isoform 1)], was produced synthetically. Chemical synthesis was achieved by native ligation from two separate domains yielding a bioactive peptide that reflected the characteristics of native penaeidin. Synthetic Pen4-1 proved to be an effective antimicrobial peptide, particularly against the broad-spectrum pathogen Fusarium oxysporum, exhibiting a complex effect on reproductive growth at inhibitory concentrations resulting in the suppression of spore formation. Pen4-1 exhibits unique features [not previously observed for penaeidins from the Pacific white shrimp (L. vannamei)], including target-species specificity against Gram-positive bacteria, indicating a potential partitioning of antimicrobial function among this family of peptides. The proline-rich domain of penaeidin class 4 alone was an active antimicrobial peptide, having the same target range as the full-length Pen4-1. These findings indicate that the proline-rich domain of penaeidin is sufficient to confer target specificity and that divergence in this domain between classes can result in a gain in antimicrobial function as observed for the proline-rich domain of Pen4-1.

Cyclization of pyrrhocoricin retains structural elements crucial for the antimicrobial activity of the native peptide

Pyrrhocoricin is a naturally occurring antimicrobial peptide from the European fire bug Pyrrhocoris apterus. It has submicromolar activity against a range of Gram-negative bacterial strains and has created recent interest as a lead for the development of novel antibiotic compounds. In this study, we have used NMR spectroscopy to determine the solution structures of pyrrhocoricin and a synthetic macrocyclic derivative that has improved in vivo pharmaceutical properties. Native pyrrhocoricin is largely disordered in solution, but there is evidence of a subpopulation with ordered turn regions over residues 2-5, 4-7, and 16-19. The macrocyclic derivative incorporates a nine amino acid linker joining the N- and C-termini, which does not adversely affect the antimicrobial potency but leads to a broader spectrum of activity. The NMR data suggest that the turn conformations in the cyclic derivative are similar to those in the native form, thus implicating them in the biological function.

Defensins and Other Antimicrobial Peptides at the Ocular Surface

Although constantly exposed to the environment and "foreign bodies" such as contact lenses and unwashed fingertips, the ocular surface succumbs to infection relatively infrequently. This is, in large part, due to a very active and robust innate immune response mounted at the ocular surface. Studies over the past 20 years have revealed that small peptides with antimicrobial activity are a major component of the human innate immune response system. The ocular surface is no exception, with peptides of the defensin and cathelicidin families being detected in the tear film and secreted by corneal and conjunctival epithelial cells. There is also much evidence to suggest that the role of some antimicrobial peptides is not restricted to direct killing of pathogens, but, rather, that they function in various aspects of the immune response, including recruitment of immune cells, and through actions on dendritic cells provide a link to adaptive immunity. A role in wound healing is also supported. In this article, the properties, mechanisms of actions and functional roles of antimicrobial peptides are reviewed, with particular emphasis on the potential multifunctional roles of defensins and LL-37 (the only known human cathelicidin) at the ocular surface.

An entomopathogenic bacterium, Xenorhabdus nematophila, inhibits the expression of an antibacterial peptide, cecropin, of the beet armyworm, Spodoptera exigua

An entomopathogenic bacterium, Xenorhabdus nematophila, is known to depress hemocyte nodule formation of target insects by inhibiting eicosanoid biosynthesis. This study analyzed the inhibitory effect of X. nematophila on the humoral immunity of the target insects and tested its association with the host eicosanoid pathway. Plasma collected from the fifth instar larvae of Spodoptera exigua, when they were injected with X. nematophila, did not show antibacterial activity against Escherichia coli by a growth inhibition zone assay. In comparison, heat-killed X. nematophila induced significant antibacterial activity in the plasma. The antibacterial humoral activity was further demonstrated by examining a specific potent antibacterial peptide, cecropin. Two cecropin genes ('A' and 'B') were partially cloned from the fifth instar larvae of S. exigua by conserved degenerate primers using nested reverse transcriptase-polymerase chain reaction (RT-PCR). They showed high homologies with known cecropins from other lepidopteran species. Northern analysis using the cecropin probe showed that the injection of the heat-killed X. nematophila induced significant expression of a cecropin mRNA transcript (approximately 1.1 kb), but the larvae injected with the live bacteria did not show the corresponding transcript. Injection of arachidonic acid did not rescue the inhibition of X. nematophila based on either antibacterial activity or cecropin gene expression. The addition of dexamethasone, a specific phospholipase A2 inhibitor, did not inhibit antibacterial activity or cecropin gene expression when the larvae were injected with heat-killed X. nematophila. These results suggest that X. nematophila inhibits the antibacterial humoral immune reaction as well as the cellular immune reaction in S. exigua and that the inhibition of X. nematophila on the expression of the antibacterial peptide is not associated with inhibition of the eicosanoid pathway.

Acylation of SC4 dodecapeptide increases bactericidal potency against Gram-positive bacteria, including drug-resistant strains

We have conjugated dodecyl and octadecyl fatty acids to the N-terminus of SC4, a potently bactericidal, helix-forming peptide 12-mer (KLFKRHLKWKII), and examined the bactericidal activities of the resultant SC4 'peptide-amphiphile' molecules. SC4 peptide-amphiphiles showed up to a 30-fold increase in bactericidal activity against Gram-positive strains (Staphylococcus aureus, Streptococcus pyogenes and Bacillus anthracis), including S. aureus strains resistant to conventional antibiotics, but little or no increase in bactericidal activity against Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa). Fatty acid conjugation improved endotoxin (lipopolysaccharide) neutralization by 3- to 6-fold. Although acylation somewhat increased lysis of human erythrocytes, it did not increase lysis of endothelial cells, and the haemolytic effects occurred at concentrations 10- to 100-fold higher than those required for bacterial cell lysis. For insight into the mechanism of action of SC4 peptide-amphiphiles, CD, NMR and fluorescence spectroscopy studies were performed in micelle and liposome models of eukaryotic and bacterial cell membranes. CD indicated that SC4 peptide-amphiphiles had the strongest helical tendencies in liposomes mimicking bacterial membranes, and strong membrane integration of the SC4 peptide-amphiphiles was observed using tryptophan fluorescence spectroscopy under these conditions; results that correlated with the increased bactericidal activities of SC4 peptide-amphiphiles. NMR structural analysis in micelles demonstrated that the two-thirds of the peptide closest to the fatty acid tail exhibited a helical conformation, with the positively-charged side of the amphipathic helix interacting more with the model membrane surface. These results indicate that conjugation of a fatty acid chain to the SC4 peptide enhances membrane interactions, stabilizes helical structure in the membrane-bound state and increases bactericidal potency.

The insect antimicrobial peptide,l-pyrrhocoricin, binds to and stimulates the ATPase activity of both wild-type and lidless DnaK

Recent reports have indicated that insect antimicrobial peptides kill bacteria by inhibiting the molecular chaperone DnaK. It was proposed that the antimicrobial peptide, all-L-pyrrhocoricin (L-PYR), binds to two sites on DnaK, the conventional substrate-binding site and the multi-helical C-terminal lid, and that inhibition of DnaK comes about from the lid mode of binding. In this report, we show using two different assays that L-PYR binds to and stimulates the ATPase activity of both wild-type and a lidless variant of DnaK. Our study shows that L-PYR interacts with DnaK much like the all-L NR (NRLLLTG) peptide, which is known to bind in the conventional substrate-binding site of DnaK. L-PYR antimicrobial activity is thus a consequence of the competitive inhibition of bacterial DnaK.

Arthropod and mollusk defensins--evolution by exon-shuffling

Arthropod and mollusk defensins are secreted antibacterial proteins that exhibit similarity in sequence, mode of action and structure and are expressed ubiquitously. Comparison of the gene organization of a newly cloned scorpion defensin gene, with that of other arthropods and the mussel, revealed that all exons and introns, aside from the exon encoding the mature protein, differ widely in number, size and sequence. This variability suggests that the exon encoding the mature defensin has undergone exon-shuffling and integrated downstream of unrelated leader sequences during evolution. Unlike other exon-shuffling events, in which modules are added into existing proteins, arthropod and mollusk defensins represent the first instance of exon-shuffling of autonomous modules.

Identification of 1-lysophosphatidylethanolamine (C(16:1)) as an antimicrobial compound in the housefly, Musca domestica

We observed that a methanolic whole body extract of uninfected last instar larvae of the housefly, Musca domestica, displayed antifungal and antibacterial activity. We have further purified this extract to a single active fraction using reversed phase high performance liquid chromatography. The pure fraction inhibited growth of the Gram-positive bacteria Bacillus thuringiensis and the yeast Saccharomyces cerevisiae, but not the Gram-negative bacteria Escherichia coli. The active compound was determined to have a molecular mass of 451.2 Da. Further analysis by nuclear magnetic resonance identified the substance as mono-unsaturated 1-lysophosphatidylethanolamine (C(16:1)) (1-LPE). The structurally different and more common 2-LPE have been described as mediators of the antimicrobial activity of rimenophenazine antibiotic agents (Van Rensburg et al., 1992). Our results suggest that the isolated 1-LPE displays a higher activity in comparison, possibly based on structure-specific differences in activity.

Immune stimulation in the silkworm, Bombyx mori L., by CpG oligodeoxynucleotides

Synthetic ODNs containing unmethylated CpG dinucleotides are known to stimulate immune responses in vertebrates, but so far the effect has not been studied in insects. In this report, we describe an induction of immune response following injection of oligodeoxynucleotides (ODNs) into the insect hemocoel. The fifth instar silkworm (Bombyx mori L.) larvae were injected with several synthetic ODNs containing variable number of unmethylated CpG motifs, heat-denatured genomic DNA of B. mori itself, or intact genomic DNA to observe a new induction pattern in the insect immune mechanism. When the induction of immune response was examined based on the expression rates of genes for antibacterial peptides such as attacin and cecropin, we could confirm that it was triggered upon injection of ODNs. The expression was, however, neither dependent on numbers of CpG motifs nor methylation of CpGs in ODNs. Furthermore, it was confirmed that the presence of CpG in ODN was not involved in the induction pattern of insect immunity caused by ODNs, although it has been reported that vertebrates respond in a specific manner against invading ODNs containing CpG dinucleotides. In addition, insect immunity was not stimulated by injection of intact DNA from host. In contrast, the injection of denatured genomic DNA provoked the host immune reaction. Taken together, our data suggest that foreignness of ODNs or DNA might be a key factor in the induction of insect immunity.

Microcin E492 antibacterial activity: evidence for a TonB-dependent inner membrane permeabilization on Escherichia coli

The mechanism of action of microcin E492 (MccE492) was investigated for the first time in live bacteria. MccE492 was expressed and purified to homogeneity through an optimized large-scale procedure. Highly purified MccE492 showed potent antibacterial activity at minimal inhibitory concentrations in the range of 0.02-1.2 microM. The microcin bactericidal spectrum of activity was found to be restricted to Enterobacteriaceae and specifically directed against Escherichia and Salmonella species. Isogenic bacteria that possessed mutations in membrane proteins, particularly of the TonB-ExbB-ExbD complex, were assayed. The microcin bactericidal activity was shown to be TonB- and energy-dependent, supporting the hypothesis that the mechanism of action is receptor mediated. In addition, MccE492 depolarized and permeabilized the E. coli cytoplasmic membrane. The membrane depolarization was TonB dependent. From this study, we propose that MccE492 is recognized by iron-siderophore receptors, including FepA, which promote its import across the outer membrane via a TonB- and energy-dependent pathway. MccE492 then inserts into the inner membrane, whereupon the potential becomes destabilized by pore formation. Because cytoplasmic membrane permeabilization of MccE492 occurs beneath the threshold of the bactericidal concentration and does not result in cell lysis, the cytoplasmic membrane is not hypothesized to be the sole target of MccE492.

Biological activities of synthetic analogs of halocidin, an antimicrobial peptide from the tunicate Halocynthia aurantium

Halocidin is a heterodimer antimicrobial peptide previously isolated from the tunicate Halocynthia aurantium. Based on the larger monomer (18Hc) of halocidin, nine halocidin congeners, including a series of 6 peptides truncated successively from the carboxyl-terminal end of 18Hc and 3 analogs (18HcKK, K19Hc, and K19HcKK), which have lysine residues in place of two internal histidines or have a lysine added to the amino terminus of the 18Hc molecule, were prepared. Each peptide was also converted into a homodimeric version. The antimicrobial activities of halocidin congeners truncated from the C terminus were dramatically decreased, suggesting that the full length of 18Hc is required for maintaining its maximum antimicrobial activity. Dimer forms of halocidin congeners exhibited stronger antimicrobial activities than the monomer of the corresponding peptide. Four dimer peptides (di-18Hc, di-18HcKK, di-K19Hc, and di-K19HcKK) were analyzed for antimicrobial activities against 10 clinically isolated antibiotic-resistant bacteria in elevated concentrations of NaCl or MgCl(2). Of the peptides studied here, di-K19Hc retained invariably strong activity against all bacteria in diverse conditions and also showed much reduced hemolytic activity against human erythrocytes.

A Leu-Lys-rich antimicrobial peptide: activity and mechanism

To develop novel antibiotic peptides useful as therapeutic drugs, the analogues were designed to increase not only net positive charge by Lys substitution but also hydrophobic helix region by Leu substitution from cecropin A (1-8)-magainin 2 (1-12) hybrid peptide (CA-MA). In particular, CA-MA analogue P5 (P5), designed by flexible region (GIG-->P) substitution, Lys (positions 4, 8, 14, 15) and Leu (positions 5, 6, 12, 13, 16, 17, 20) substitutions, showed an enhanced antimicrobial and antitumor activity without hemolysis. Confocal microscopy showed that P5 was located in the plasma membrane. The antibacterial effects of analogues were further confirmed by using 1,6-diphenyl-1,3,5-hexatriene as a plasma membrane probe. Flow cytometric analysis revealed that P5 acted in an energy-independent manner. This interaction is also independent of the ionic environment. Furthermore, P5 causes significant morphological alterations of the bacterial surfaces as shown by scanning electron microscopy and showed strong membrane disrupting activity when examined using liposomes (phosphatidyl choline/cholesterol; 10:1, w/w). Its potent antibiotic activity suggests that P5 is an excellent candidate as a lead compound for the development of novel antiinfective agents.

Insect immunity and its implication in mosquito-malaria interactions

Insects' resistance to infectious agents is essential for their own survival and also for the health of the plant, animal and human populations with which they closely interact. Several of the major human diseases are spread by insects and are rapidly expanding as a result of the development of insecticide resistance in vectors and drug resistance in parasites. A vector insects' permissiveness to a pathogen, and hence the spread of the disease, will largely depend on the compatibility of the molecular interactions between the two species and the capability of the insect immune system to recognize and kill the pathogen. The innate immune system comprises a variety of components and mechanisms that can discriminate between different microorganisms and mount specific responses to control pathogenic infections. An impressive body of knowledge on the insects' innate immunity has been generated from studies in the model organism Drosophila. These studies are now guiding the exploration of the immune system in the vector mosquito of human malaria, Anopheles, and its implication in the elimination of parasites. Anopheles immune responses have been linked to parasite losses and some refractory mosquitoes can kill all parasites through specific defence mechanisms. The recently sequenced Drosophila and Anopheles genomes provide a detailed and comparative view on their immune gene repertoires that in combination with post-genomic analyses is used to further dissect the complex mechanisms of Plasmodium killing in the mosquito.

Development of novel antibacterial peptides that kill resistant isolates

The rapid emergence of bacterial strains that are resistant to current antibiotics requires the development of novel types of antimicrobial compounds. Proline-rich cationic antibacterial peptides such as pyrrhocoricin kill responsive bacteria by binding to the 70 kDa heat shock protein DnaK and inhibiting protein folding. We designed and synthesized multiply protected dimeric analogs of pyrrhocoricin and optimized the in vitro antibacterial efficacy assays for peptide antibiotics. Pyrrhocoricin and the designed dimers killed beta-lactam, tetracycline- or aminoglycoside-resistant strains of Escherichia coli, Salmonella typhimurium, Klebsiella pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis in the submicromolar or low micromolar concentration range. One of the peptides also killed Pseudomonas aeruginosa. The designed dimers showed improved stability in mammalian sera compared to the native analog. In a murine H. influenzae lung infection model, a single dose of a dimeric pyrrhocoricin analog reduced the bacteria in the bronchoalveolar lavage when delivered intranasally. The solid-phase synthesis was optimized for large-scale laboratory preparations.

Temporin L: antimicrobial, haemolytic and cytotoxic activities, and effects on membrane permeabilization in lipid vesicles

The temporins are a family of small, linear antibiotic peptides with intriguing biological properties. We investigated the antibacterial, haemolytic and cytotoxic activities of temporin L (FVQWFSKFLGRIL-NH2), isolated from the skin of the European red frog Rana temporaria. The peptide displayed the highest activity of temporins studied to date, against both human erythrocytes and bacterial and fungal strains. At variance with other known temporins, which are mainly active against Gram-positive bacteria, temporin L was also active against Gram-negative strains such as Pseudomonas aeruginosa A.T.C.C. 15692 and Escherichia coli D21 at concentrations comparable with those that are microbiocidal to Gram-positive bacteria. In addition, temporin L was cytotoxic to three different human tumour cell lines (Hut-78, K-562 and U-937), causing a necrosis-like cell death, although sensitivity to the peptide varied markedly with the specific cell line tested. A study of the interaction of temporin L with liposomes of different lipid compositions revealed that the peptide causes perturbation of bilayer integrity of both neutral and negatively charged membranes, as revealed by the release of a vesicle-encapsulated fluorescent marker, and that the action of the peptide is modulated to some extent by membrane lipid composition. In particular, the presence of negatively charged lipids in the model bilayer inhibits the lytic power of temporin L. We also show that the release of fluorescent markers caused by temporin L is size-dependent and that the peptide does not have a detergent-like effect on the membrane, suggesting that perturbation of bilayer organization takes place on a local scale, i.e. through the formation of pore-like openings.

Antimicrobial peptides: synthesis and antibacterial activity of linear and cyclic drosocin and apidaecin 1b analogues

Drosocin and apidaecin Ib are two insect antimicrobial peptides showing a significant sequence homology and a common mechanism of action, which includes stereoselective elements but is devoid of any pore-forming activity. A substantial difference between the two peptides is the presence in the drosocin sequence of an O-glycosylated threonine residue, which is important for its antimicrobial activity. Through the synthesis of a series of differently glycosylated drosocin analogues, we have shown that the antimicrobial activity against several Gram-negative bacteria appears to be modulated by the sugar moiety (Gal vs GalNAc) and the type of glycosidic linkage (alpha-O-, beta-O-, or alpha-C-). The insertion of a glycosylated threonine residue in the apidaecin Ib sequence improves the sequence homology with drosocin but reduces the antimicrobial activity. To gain information on the possible bioactive conformation of these peptides, we synthesized an unglycosylated cyclic analogue of drosocin, containing an intrachain disulfide bond, and the head-to-tail cyclic analogues of drosocin and apidaecin, as well as their corresponding cyclic dimers. Only the large cyclic dimer of apidaecin partially retained the antimicrobial activity, suggesting that a bending of the peptide chain, in particular in the middle of the molecule, is not a structural element characteristic of the bioactive conformation of drosocin and apidaecin. Experiments aimed at testing the effect of selected drosocin and apidaecin peptides on biological membranes showed that some peptides display a moderate hemolytic activity and that a dissociation between antibacterial activity and cytotoxicity to eukaryotic cells can be achieved in differently glycosylated peptide analogues.

Identification of crucial residues for the antibacterial activity of the proline-rich peptide, pyrrhocoricin

Members of the proline-rich antibacterial peptide family, pyrrhocoricin, apidaecin and drosocin appear to kill responsive bacterial species by binding to the multihelical lid region of the bacterial DnaK protein. Pyrrhocoricin, the most potent among these peptides, is nontoxic to healthy mice, and can protect these animals from bacterial challenge. A structure-antibacterial activity study of pyrrhocoricin against Escherichia coli and Agrobacterium tumefaciens identified the N-terminal half, residues 2-10, the region responsible for inhibition of the ATPase activity, as the fragment that contains the active segment. While fluorescein-labeled versions of the native peptides entered E. coli cells, deletion of the C-terminal half of pyrrhocoricin significantly reduced the peptide's ability to enter bacterial or mammalian cells. These findings highlighted pyrrhocoricin's suitability for combating intracellular pathogens and raised the possibility that the proline-rich antibacterial peptides can deliver drug leads into mammalian cells. By observing strong relationships between the binding to a synthetic fragment of the target protein and antibacterial activities of pyrrhocoricin analogs modified at strategic positions, we further verified that DnaK was the bacterial target macromolecule. Inaddition, the antimicrobial activity spectrum of native pyrrhocoricin against 11 bacterial and fungal strains and the binding of labeled pyrrhocoricin to synthetic DnaK D-E helix fragments of the appropriate species could be correlated. Mutational analysis on a synthetic E. coli DnaK fragment identified a possible binding surface for pyrrhocoricin.

Defensins of vertebrate animals

During the past year, novel beta-defensins of mice and men have been identified, together with a novel defensin subfamily (the circular or 'theta' minidefensins) in primates. Insight into the evolution of defensins has been obtained from structural studies, and several mechanisms related to microbial resistance to defensins have been delineated. There is now convincing evidence that defensins augment adaptive immune responses.

Salicylic acid and NIM1/NPR1-independent gene induction by incompatible Peronospora parasitica in arabidopsis

To identify pathogen-induced genes distinct from those involved in systemic acquired resistance, we used cDNA-amplified fragment length polymorphism to examine RNA levels in Arabidopsis thaliana wild type, nim1-1, and salicylate hydroxylase-expressing plants after inoculation with an incompatible isolate of the downy mildew pathogen Peronospora parasitica. Fifteen genes are described, which define three response profiles on the basis of whether their induction requires salicylic acid (SA) accumulation and NIM1/NPR1 activity, SA alone, or neither. Sequence analysis shows that the genes include a calcium binding protein related to TCH3, a protein containing ankyrin repeats and potential transmembrane domains, three glutathione S-transferase gene family members, and a number of small, putatively secreted proteins. We further characterized this set of genes by assessing their expression patterns in each of the three plant lines after inoculation with a compatible P. parasitica isolate and after treatment with the SA analog 2,6-dichloroisonicotinic acid. Some of the genes within subclasses showed different requirements for SA accumulation and NIM1/NPR1 activity, depending upon which elicitor was used, indicating that those genes were not coordinately regulated and that the regulatory pathways are more complex than simple linear models would indicate.

N-terminal fatty acid substitution increases the leishmanicidal activity of CA(1-7)M(2-9), a cecropin-melittin hybrid peptide

In order to improve the leishmanicidal activity of the synthetic cecropin A-melittin hybrid peptide CA(1-7)M(2-9) (KWKLFKKIGAVLKVL-NH(2)), a systematic study of its acylation with saturated linear fatty acids was carried out. Acylation of the N(epsilon)-7 lysine residue led to a drastic decrease in leishmanicidal activity, whereas acylation at lysine 1, in either the alpha or the epsilon NH(2) group, increased up to 3 times the activity of the peptide against promastigotes and increased up to 15 times the activity of the peptide against amastigotes. Leishmanicidal activity increased with the length of the fatty acid chain, reaching a maximum for the lauroyl analogue (12 carbons). According to the fast kinetics, dissipation of membrane potential, and parasite membrane permeability to the nucleic acid binding probe SYTOX green, the lethal mechanism was directly related to plasma membrane permeabilization.