New caerin antibacterial peptides from the skin glands of the Australian tree frog Litoria xanthomera

Proteomic analysis of anti-MRSA activity of caerin 1.1/1.9 in a murine skin infection model and their in vitro anti-biofilm effects against Acinetobacter baumannii

IMPORTANCE

Caerin 1.1 and caerin 1.9, natural antimicrobial peptides derived from tree frogs, have demonstrated the ability to inhibit the growth of antibiotic-resistant bacteria, comparable to certain widely used antibiotics. Additionally, these peptides exhibit the capacity to prevent or treat biofilms formed by bacteria in conjunction with bodily components. The mechanisms underlying their antibacterial effects were investigated through a mouse model of bacterial skin infection, utilizing proteomic analysis as a technological approach.

Tandem Mass Spectrometry de novo Sequencing of the Skin Defense Peptides of the Central Slovenian Agile Frog Rana dalmatina

Peptides released on frogs' skin in a stress situation represent their only weapon against micro-organisms and predators. Every species and even population of frog possesses its own peptidome being appropriate for their habitat. Skin peptides are considered potential pharmaceuticals, while the whole peptidome may be treated as a taxonomic characteristic of each particular population. Continuing the studies on frog peptides, here we report the peptidome composition of the Central Slovenian agile frog Rana dalmatina population. The detection and top-down de novo sequencing of the corresponding peptides was conducted exclusively by tandem mass spectrometry without using any chemical derivatization procedures. Collision-induced dissociation (CID), higher energy collision-induced dissociation (HCD), electron transfer dissociation (ETD) and combined MS3 method EThcD with stepwise increase of HCD energy were used for that purpose. MS/MS revealed the whole sequence of the detected peptides including differentiation between isomeric Leu/Ile, and the sequence portion hidden in the disulfide cycle. The array of the discovered peptide families (brevinins 1 and 2, melittin-related peptides (MRPs), temporins and bradykinin-related peptides (BRPs)) is quite similar to that of R. temporaria. Since the genome of this frog remains unknown, the obtained results were compared with the recently published transcriptome of R. dalmatina.

Caerin 1.1 and 1.9 Peptides from Australian Tree Frog Inhibit Antibiotic-Resistant Bacteria Growth in a Murine Skin Infection Model

The host defense peptide caerin 1.9 was originally isolated from skin secretions of an Australian tree frog and inhibits the growth of a wide range of bacteria in vitro. In this study, we demonstrated that caerin 1.9 shows high bioactivity against several bacteria strains, such as Staphylococcus aureus, Acinetobacter baumannii, methicillin-resistant Staphylococcus aureus (MRSA), and Streptococcus haemolyticus in vitro. Importantly, unlike the antibiotic Tazocin, caerin 1.9 does not induce bacterial resistance after 30 rounds of in vitro culture. Moreover, caerin 1.1, another peptide of the caerin family, has an additive antibacterial effect when used together with caerin 1.9. Furthermore, caerin 1.1 and 1.9 prepared in the form of a temperature-sensitive gel inhibit MRSA growth in a skin bacterial infection model of two murine strains. These results indicate that caerin 1.1 and 1.9 peptides could be considered an alternative for conventional antibiotics. IMPORTANCE Antibiotic-resistant bacteria cause severe problems in the clinic. We show in our paper that two short peptides isolated from an Australian frog and prepared in the form of a gel are able to inhibit the growth of antibiotic-resistant bacteria in mice, and, unlike antibiotics, these peptides do not lead to the development of peptide-resistant bacteria strains.

Mg alloy surface immobilised with caerin peptides acquires enhanced antibacterial ability and putatively improved corrosion resistance

Magnesium (Mg) has mechanical properties similar to human bones and Mg alloy is considered ideal medical implant material. However, the high velocity of degradation inside the human inner environment severely hampers the usage of Mg alloys. In this study, caerin peptide 1.9 (F3) and a modified sequence of caerin 1.1 (F1) with anti-bacterial activity, were covalently immobilised on the surface of Mg alloys by plasma chemical click reaction. The in vitro antibacterial activity and corrosion resistance of these caerin peptide-immobilised Mg alloys were investigated in Dulbecco's Modified Eagle Medium (DMEM) solution. Un-immobilised Mg alloy sample, blank drug-sensitive tablet (BASD) and a commonly used antibiotics Tazocin were used for comparison. Results showed that peptide immobilised Mg samples showed observable improved corrosion resistance and prolonged antibacterial effect compared to non-immobilised Mg alloy and free caerin peptides. These results indicate that coating Mg alloy with caerin peptides obviously increases the alloy's antibacterial ability and putatively improves the corrosion resistance in vitro. The mechanism underlying the prolonged antibacterial effect for annealed Mg alloys immobilised with the peptides (especially F3) remains unclear, which worth further experimental and theoretical investigation.

In Silico Discovery of Antimicrobial Peptides as an Alternative to Control SARS-CoV-2

A serious pandemic has been caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The interaction between spike surface viral protein (Sgp) and the angiotensin-converting enzyme 2 (ACE2) cellular receptor is essential to understand the SARS-CoV-2 infectivity and pathogenicity. Currently, no drugs are available to treat the infection caused by this coronavirus and the use of antimicrobial peptides (AMPs) may be a promising alternative therapeutic strategy to control SARS-CoV-2. In this study, we investigated the in silico interaction of AMPs with viral structural proteins and host cell receptors. We screened the antimicrobial peptide database (APD3) and selected 15 peptides based on their physicochemical and antiviral properties. The interactions of AMPs with Sgp and ACE2 were performed by docking analysis. The results revealed that two amphibian AMPs, caerin 1.6 and caerin 1.10, had the highest affinity for Sgp proteins while interaction with the ACE2 receptor was reduced. The effective AMPs interacted particularly with Arg995 located in the S2 subunits of Sgp, which is key subunit that plays an essential role in viral fusion and entry into the host cell through ACE2. Given these computational findings, new potentially effective AMPs with antiviral properties for SARS-CoV-2 were identified, but they need experimental validation for their therapeutic effectiveness.

Bioinformatic Analysis of 1000 Amphibian Antimicrobial Peptides Uncovers Multiple Length-Dependent Correlations for Peptide Design and Prediction

Amphibians are widely distributed on different continents, except for the polar regions. They are important sources for the isolation, purification and characterization of natural compounds, including peptides with various functions. Innate immune antimicrobial peptides (AMPs) play a critical role in warding off invading pathogens, such as bacteria, fungi, parasites, and viruses. They may also have other biological functions such as endotoxin neutralization, chemotaxis, anti-inflammation, and wound healing. This article documents a bioinformatic analysis of over 1000 amphibian antimicrobial peptides registered in the Antimicrobial Peptide Database (APD) in the past 18 years. These anuran peptides were discovered in Africa, Asia, Australia, Europe, and America from 1985 to 2019. Genomic and peptidomic studies accelerated the discovery pace and underscored the necessity in establishing criteria for peptide entry into the APD. A total of 99.9% of the anuran antimicrobial peptides are less than 50 amino acids with an average length of 24 and a net charge of +2.5. Interestingly, the various amphibian peptide families (e.g., temporins, brevinins, esculentins) can be connected through multiple length-dependent relationships. With an increase in length, peptide net charge increases, while the hydrophobic content decreases. In addition, glycine, leucine, lysine, and proline all show linear correlations with peptide length. These correlations improve our understanding of amphibian peptides and may be useful for prediction and design of new linear peptides with potential applications in treating infectious diseases, cancer and diabetes.

TC38, a teleost TFPI-2 peptide that kills bacteria via penetration of the cell membrane and interaction with nucleic acids

Tissue factor pathway inhibitor 2 (TFPI-2) is an analog of TFPI-1 and a potent endogenous inhibitor of tissue factor (TF)-mediated blood coagulation. Recent reports have proven that the C-terminal of TFPI-2 peptides in humans and several other vertebrates possesses antibacterial activity against Gram-positive and Gram-negative bacteria. In our previous study, we reported that the TFPI-2 peptide, TC38 in tongue sole (Cynoglossus semilaevis) was active against Micrococcus luteus. In this study, we further examine the antimicrobial spectrum, mechanism of action, and function of TC38 in tongue sole. Our results indicate that TC38 is active against the Gram-negative bacteria Vibrio ichthyoenteri, Vibrio litoralis, Vibrio parahaemolyticus, and Vibrio vulnificus, as well as the fish Megalocytivirus, infectious spleen and kidney necrosis virus (ISKNV). The mechanism of action of TC38 against V. vulnificus was explored. The results showed that TC38 killed V. vulnificus cells without lysis of the cell membrane. FITC-labeled TC38 was able to penetrate the cell membrane and bind to DNA and RNA, then disrupt cellular function, eventually leading to cell death. Administration of TC38 to tongue sole significantly improved its defense against V. vulnificus infection. Overall, these results indicate that TC38 is a novel peptide with a broad antimicrobial spectrum. Furthermore, the unique action of TC38 against V. vulnificus adds new insights to the mechanism of action of vertebrate TFPI peptides. Moreover, TC38 is an interesting antimicrobial agent that could be useful in the fight against pathogenic invasion in aquaculture.

Proteolytic degradation and deactivation of amphibian skin peptides obtained by electrical stimulation of their dorsal glands

Amphibians are among the oldest creatures on our planet. Their only defensive weapon efficient against microorganisms and predators involves their skin secretion. The wide range of biological activities of the peptides in the skin secretion of amphibians makes these compounds rather interesting for generation of prospective pharmaceuticals. The first step in studying these molecules requires their structures to be established. Mass spectrometry is the most powerful tool for this purpose. The sampling and sample preparation stages preceding mass spectrometry experiments appear to be rather crucial. The results obtained here demonstrate that these preparation procedures might lead to partial or complete loss of the bioactive peptides in the secretion. Five minutes in water was enough to completely destroy all of the bioactive peptides in the skin secretion of the marsh frog (Rana ridibunda); even immediate addition of methanol to the water solution of the peptides did not prevent partial destruction. Concerted effort should be directed towards development of the most efficient procedure to keep the secreted peptides intact. Graphical Abstract ᅟ.

Development of antimicrobial peptide defenses of southern leopard frogs, Rana sphenocephala, against the pathogenic chytrid fungus, Batrachochytrium dendrobatidis

Amphibian species face the growing threat of extinction due to the emerging fungal pathogen Batrachochytrium dendrobatidis, which causes the disease chytridiomycosis. Antimicrobial peptides (AMPs) produced in granular glands of the skin are an important defense against this pathogen. Little is known about the ontogeny of AMP production or the impact of AMPs on potentially beneficial symbiotic skin bacteria. We show here that Rana (Lithobates) sphenocephala produces a mixture of four AMPs with activity against B. dendrobatidis, and we report the minimum inhibitory concentration (MIC) of synthesized replicates of these four AMPs tested against B. dendrobatidis. Using mass spectrometry and protein quantification assays, we observed that R. sphenocephala does not secrete a mature suite of AMPs until approximately 12 weeks post-metamorphosis, and geographically disparate populations produce a different suite of peptides. Use of norepinephrine to induce maximal secretion significantly reduced levels of culturable skin bacteria.

Nikkomycin Z is an effective inhibitor of the chytrid fungus linked to global amphibian declines

Fungal infections in humans, wildlife, and plants are a growing concern because of their devastating effects on human and ecosystem health. In recent years, populations of many amphibian species have declined, and some have become extinct due to chytridiomycosis caused by the fungal pathogen Batrachochytrium dendrobatidis. For some endangered amphibian species, captive colonies are the best intermediate solution towards eventual reintroduction, and effective antifungal treatments are needed to cure chytridiomycosis and limit the spread of this pathogen in such survival assurance colonies. Currently, the best accepted treatment for infected amphibians is itraconazole, but its toxic side effects reduce its usefulness for many species. Safer antifungal treatments are needed for disease control. Here, we show that nikkomycin Z, a chitin synthase inhibitor, dramatically alters the cell wall stability of B. dendrobatidis cells and completely inhibits growth of B. dendrobatidis at 250 μM. Low doses of nikkomycin Z enhanced the effectiveness of natural antimicrobial skin peptide mixtures tested in vitro. These studies suggest that nikkomycin Z would be an effective treatment to significantly reduce the fungal burden in frogs infected by B. dendrobatidis.

Immune evasion or avoidance: fungal skin infection linked to reduced defence peptides in Australian green-eyed treefrogs, Litoria serrata

Many parasites and pathogens suppress host immunity to maintain infection or initiate disease. On the skin of many amphibians, defensive peptides are active against the fungus Batrachochytrium dendrobatidis (Bd), the causative agent of the emerging infectious disease chytridiomycosis. We tested the hypothesis that infection with the fungus may be linked to lower levels of defensive peptides. We sampled both ambient (or constitutive) skin peptides on the ventral surface immediately upon capture, and stored skin peptides induced from granular glands by norepinephrine administration of Australian green-eyed treefrogs, Litoria serrata. Upon capture, uninfected frogs expressed an array of antimicrobial peptides on their ventral surface, whereas infected frogs had reduced skin peptide expression. Expression of ambient skin peptides differed with infection status, and antimicrobial peptides maculatin 1.1 and 2.1 were on average three times lower on infected frogs. However, the repertoire of skin peptides stored in granular glands did not differ with infection status; on average equal quantities were recovered from infected and from uninfected frogs. Our results could have at least two causes: (1) frogs with reduced peptide expression are more likely to become infected; (2) Bd infection interferes with defence peptides by inhibiting release or causing selective degradation of peptides on the skin surface. Immune evasion therefore may contribute to the pathogenesis of chytridiomycosis and a mechanistic understanding of this fungal strategy may lead to improved methods of disease control.

Host-defense peptides of Australian anurans. Part 2. Structure, activity, mechanism of action, and evolutionary significance

A previous review summarized research prior to 2004 carried out on the bioactive host-defense peptides contained in the skin secretions of Australian anurans (frogs and toads). This review covers the extension of that research from 2004 to 2012, and includes membrane-active peptides (including antibacterial, anticancer, antifungal and antiviral peptides) together with the mechanisms by which these peptides interact with model membranes, peptides that may be classified as "neuropeptides" (including smooth muscle active peptides, opioids and immunomodulators) and peptides which inhibit the formation of nitric oxide from neuronal nitric oxide synthase. The review discusses the outcome of cDNA sequencing of signal-spacer-active peptides from an evolutionary viewpoint, and also lists those peptides for which activities have not been found to this time.

Norepinephrine depletion of antimicrobial peptides from the skin glands of Xenopus laevis

The dermal granular glands of the South African clawed frog, Xenopus laevis, contain antimicrobial peptides (AMPs) that are secreted following local nerve stimulation. These natural antibiotics are active against bacteria and fungi including Batrachochytrium dendrobatidis, a fungal pathogen that causes the skin disease chytridiomycosis. Granular gland secretion can be stimulated in the laboratory by norepinephrine injection. We found that two injections of 80nmol/g norepinephrine were necessary to fully deplete the AMP stores. One injection resulted in the secretion of most of the stored peptides. A second injection, 2 days later, released a small amount of additional AMPs that are not compositionally different from those released by the first injection. A third injection, 4 days after the first, did not result in further AMP release. Mass spectrometry and histology confirmed that glands are depleted after two injections. Periodic acid-Schiff staining indicated that mucus gland secretion was also induced by norepinephrine.

Solution NMR studies of amphibian antimicrobial peptides: linking structure to function?

The high-resolution three-dimensional structure of an antimicrobial peptide has implications for the mechanism of its antimicrobial activity, as the conformation of the peptide provides insights into the intermolecular interactions that govern the binding to its biological target. For many cationic antimicrobial peptides the negatively charged membranes surrounding the bacterial cell appear to be a main target. In contrast to what has been found for other classes of antimicrobial peptides, solution NMR studies have revealed that in spite of the wide diversity in the amino acid sequences of amphibian antimicrobial peptides (AAMPs), they all adopt amphipathic alpha-helical structures in the presence of membrane-mimetic micelles, bicelles or organic solvent mixtures. In some cases the amphipathic AAMP structures are directly membrane-perturbing (e.g. magainin, aurein and the rana-box peptides), in other instances the peptide spontaneously passes through the membrane and acts on intracellular targets (e.g. buforin). Armed with a high-resolution structure, it is possible to relate the peptide structure to other relevant biophysical and biological data to elucidate a mechanism of action. While many linear AAMPs have significant antimicrobial activity of their own, mixtures of peptides sometimes have vastly improved antibiotic effects. Thus, synergy among antimicrobial peptides is an avenue of research that has recently attracted considerable attention. While synergistic relationships between AAMPs are well described, it is becoming increasingly evident that analyzing the intermolecular interactions between these peptides will be essential for understanding the increased antimicrobial effect. NMR structure determination of hybrid peptides composed of known antimicrobial peptides can shed light on these intricate synergistic relationships. In this work, we present the first NMR solution structure of a hybrid peptide composed of magainin 2 and PGLa bound to SDS and DPC micelles. The hybrid peptide adopts a largely helical conformation and some information regarding the inter-helix organization of this molecule is reported. The solution structure of the micelle associated MG2-PGLa hybrid peptide highlights the importance of examining structural contributions to the synergistic relationships but it also demonstrates the limitations in the resolution of the currently used solution NMR techniques for probing such interactions. Future studies of antimicrobial peptide synergy will likely require stable isotope-labeling strategies, similar to those used in NMR studies of proteins.

A combined mass spectrometric and cDNA sequencing approach to the isolation and characterization of novel antimicrobial peptides from the skin secretions of Phyllomedusa hypochondrialis azurea

Studies conducted on amphibian skin secretions over the past 40 years have isolated and identified huge arrays of bioactive peptides, many of which have demonstrated potent anti-microbial activity. Such peptides are attracting increasing attention due to the growing problem of pathogenic microorganisms resistant to conventional antibiotics. The current study utilized a combined proteomic/genomic approach to facilitate the high throughput sequencing of five novel dermaseptins and four novel phylloseptins from the skin secretions of Phyllomedusa hypochondrialis azurea. Peptides were partially identified using Q-TOF MS/MS fragmentation and de novo sequencing, while a cDNA library was constructed from the lyophilized skin secretion. 3'-RACE reactions used primers designed for the highly conserved 5'-signal regions of previously deduced dermaseptin precursors. cDNA sequenced peptides were attributed to their respective fragmentation spectra to confirm the structure of the final processed peptides. Such an approach identified post-translational modifications in addition to deciphering isobaric amino acids. Several of the peptides were purified to homogeneity and displayed potent antimicrobial activity with minimum inhibitory concentrations starting at 0.4 microM when tested against and range of Gram-positive and Gram-negative bacteria including Escherichia coli, Staphylococcus aureus and Micrococcus luteus.

Antimicrobial peptides from amphibian skin potently inhibit human immunodeficiency virus infection and transfer of virus from dendritic cells to T cells

Topical antimicrobicides hold great promise in reducing human immunodeficiency virus (HIV) transmission. Amphibian skin provides a rich source of broad-spectrum antimicrobial peptides including some that have antiviral activity. We tested 14 peptides derived from diverse amphibian species for the capacity to inhibit HIV infection. Three peptides (caerin 1.1, caerin 1.9, and maculatin 1.1) completely inhibited HIV infection of T cells within minutes of exposure to virus at concentrations that were not toxic to target cells. These peptides also suppressed infection by murine leukemia virus but not by reovirus, a structurally unrelated nonenveloped virus. Preincubation with peptides prevented viral fusion to target cells and disrupted the HIV envelope. Remarkably, these amphibian peptides also were highly effective in inhibiting the transfer of HIV by dendritic cells (DCs) to T cells, even when DCs were transiently exposed to peptides 8 h after virus capture. These data suggest that amphibian-derived peptides can access DC-sequestered HIV and destroy the virus before it can be transferred to T cells. Thus, amphibian-derived antimicrobial peptides show promise as topical inhibitors of mucosal HIV transmission and provide novel tools to understand the complex biology of HIV capture by DCs.

Population trends associated with skin peptide defenses against chytridiomycosis in Australian frogs

Many species of amphibians in the wet tropics of Australia have experienced population declines linked with the emergence of a skin-invasive chytrid fungus, Batrachochytrium dendrobatidis. An innate defense, antimicrobial peptides produced by granular glands in the skin, may protect some species from disease. Here we present evidence that supports this hypothesis. We tested ten synthesized peptides produced by Australian species, and natural peptide mixtures from five Queensland rainforest species. Natural mixtures and most peptides tested in isolation inhibited growth of B. dendrobatidis in vitro. The three most active peptides (caerin 1.9, maculatin 1.1, and caerin 1.1) were found in the secretions of non-declining species (Litoria chloris, L. caerulea, and L. genimaculata). Although the possession of a potent isolated antimicrobial peptide does not guarantee protection from infection, non-declining species (L. lesueuri and L. genimaculata) inhabiting the rainforest of Queensland possess mixtures of peptides that may be more protective than those of the species occurring in the same habitat that have recently experienced population declines associated with chytridiomycosis (L. nannotis, L. rheocola, and Nyctimystes dayi). This study demonstrates that in vitro effectiveness of skin peptides correlates with the degree of decline in the face of an emerging pathogen. Further research is needed to assess whether this non-specific immune defense may be useful in predicting disease susceptibility in other species.

Host-defence peptides of Australian anurans: structure, mechanism of action and evolutionary significance

Host-defence peptides secreted from the skin glands of Australian frogs and toads, are, with a few notable exceptions, different from those produced by anurans elsewhere. This review summarizes the current knowledge of the following classes of peptide isolated and characterized from Australian anurans: neuropeptides (including smooth muscle active peptides, and peptides that inhibit the production of nitric oxide from neuronal nitric oxide synthase), antimicrobial and anticancer active peptides, antifungal peptides and antimalarial peptides. Other topics covered include sex pheromones of anurans, and the application of peptide profiling to (i). recognize particular populations of anurans of the same species and to differentiate between species, and (ii). investigate evolutionary aspects of peptide formation.

Molecular strategies in biological evolution of antimicrobial peptides

Gene-encoded antimicrobial peptides that protect the skin of hylid and ranin frogs against noxious microorganisms are processed from a unique family of precursor polypeptides with a unique pattern of conserved and variable regions opposite to that of conventional secreted peptides. Precursors belonging to this family, designated the preprodermaseptin, have a common N-terminal preproregion that is remarkably well conserved both within and between species, but a hypervariable C-terminal domain corresponding to antimicrobial peptides with very different lengths, sequences, charges and antimicrobial spectra. Each frog species has its own distinct panoply of 10-20 antimicrobial peptides so that the 5000 species of ranids and hylids may produce approximately 100,000 different peptide antibiotics. The strategy that these frogs have evolved to generate this enormous array of peptides includes repeated duplications of a 150 million years old ancestral gene, focal hypermutation of the antimicrobial peptide domain maybe involving a mutagenic DNA polymerase similar to Escherichia coli Pol V, and subsequent actions of positive (diversifying) selection. The hyperdivergence of skin antimicrobial peptides can be viewed as the successful evolution of a multi-drug defense system that provides frogs with maximum protection against rapidly changing microbial biota and minimizes the chance of microorganisms developing resistance to individual peptides. The impressive variations in the expression of frog skin antimicrobial peptides may be exploited for discovering new molecules and structural motifs targeting specific microorganisms for which the therapeutic armamentarium is scarce.

Interaction of antimicrobial peptides from Australian amphibians with lipid membranes

Solid-state NMR and CD spectroscopy were used to study the effect of antimicrobial peptides (aurein 1.2, citropin 1.1, maculatin 1.1 and caerin 1.1) from Australian tree frogs on phospholipid membranes. 31P NMR results revealed some effect on the phospholipid headgroups when the peptides interact with DMPC/DHPC (dimyristoylphosphatidylcholine/dihexanoylphosphatidylcholine) bicelles and aligned DMPC multilayers. 2H NMR showed a small effect of the peptides on the acyl chains of DMPC in bicelles or aligned multilayers, suggesting interaction with the membrane surface for the shorter peptides and partial insertion for the longer peptides. 15N NMR of selectively labelled peptides in aligned membranes and oriented CD spectra indicated an alpha-helical conformation with helix long axis approximately 50 degrees to the bilayer surface at high peptide concentrations. The peptides did not appear to insert deeply into PC membranes, which may explain why these positively charged peptides preferentially lyse bacterial rather than eucaryotic cells.

Amphipathic, alpha-helical antimicrobial peptides

Gene-encoded antimicrobial peptides are an important component of host defense in animals ranging from insects to mammals. They do not target specific molecular receptors on the microbial surface, but rather assume amphipathic structures that allow them to interact directly with microbial membranes, which they can rapidly permeabilize. They are thus perceived to be one promising solution to the growing problem of microbial resistance to conventional antibiotics. A particularly abundant and widespread class of antimicrobial peptides are those with amphipathic, alpha-helical domains. Due to their relatively small size and synthetic accessibility, these peptides have been extensively studied and have generated a substantial amount of structure-activity relationship (SAR) data. In this review, alpha-helical antimicrobial peptides are considered from the point of view of six interrelated structural and physicochemical parameters that modulate their activity and specificity: sequence, size, structuring, charge, amphipathicity, and hydrophobicity. It begins by providing an overview of how these vary in peptides from different natural sources. It then analyzes how they relate to the currently accepted model for the mode of action of alpha-helical peptides, and discusses what the numerous SAR studies that have been carried out on these compounds and their analogues can tell us. A comparative analysis of the many alpha-helical, antimicrobial peptide sequences that are now available then provides further information on how these parameters are distributed and interrelated. Finally, the systematic variation of parameters in short model peptides is used to throw light on their role in antimicrobial potency and specificity. The review concludes with some considerations on the potentials and limitations for the development of alpha-helical, antimicrobial peptides as antiinfective agents.

Antimicrobial peptides from amphibian skin: what do they tell us?

Amphibian skin secretions contain many biologically active compounds, such as biogenic amines, complex alkaloids, or peptides. Within the latter class of molecules, a large number of peptide antibiotics has been isolated and characterized from different amphibian species. Antimicrobial peptides are considered the effector molecules of innate immunity, acting as a first line of defense against bacterial infections, by perturbing the phospholipid bilayer of the target cell membrane. These gene-encoded molecules are synthesized as inactive precursors and in several cases their proparts were shown to have highly conserved structures. It has also been demonstrated that the promoter regions of inducible peptide antibiotics are often regulated by the transcriptional control machinery NF-kappa B/I kappa B alpha. In amphibia of Rana and Bombina genera, inhibition of transcription of the genes encoding antimicrobial peptides has been obtained by glucocorticoid treatment, which causes an increase of I kappa B alpha synthesis. Moreover, determination of the structure of a number of genes coding for antimicrobial peptides in amphibia has actually shown that their promoter regions contain recognition sites for nuclear factors.

Mode of action of linear amphipathic alpha-helical antimicrobial peptides

The increasing resistance of bacteria to conventional antibiotics resulted in a strong effort to develop antimicrobial compounds with new mechanisms of action. Antimicrobial peptides seem to be a promising solution to this problem. Many studies aimed at understanding their mode of action were described in the past few years. The most studied group includes the linear, mostly alpha-helical peptides. Although the exact mechanism by which they kill bacteria is not clearly understood, it has been shown that peptide-lipid interactions leading to membrane permeation play a role in their activity. Membrane permeation by amphipathic alpha-helical peptides can proceed via either one of the two mechanisms: (a) transmembrane pore formation via a "barrel-stave" mechanism; and (b) membrane destruction/solubilization via a "carpet-like" mechanism. The purpose of this review is to summarize recent studies aimed at understanding the mode of action of linear alpha-helical antimicrobial peptides. This review, which is focused on magainins, cecropins, and dermaseptins as representatives of the amphipathic alpha-helical antimicrobial peptides, supports the carpet-like rather the barrel-stave mechanism. That these peptides vary with regard to their length, amino acid composition, and next positive charge, but act via a common mechanism, may imply that other linear antimicrobial peptides that share the same properties also share the same mechanism.

Maculatin 1.1, an anti-microbial peptide from the Australian tree frog, Litoria genimaculata solution structure and biological activity

The dorsal glands of Australian tree frogs from the Litoria species contain a diversity of antibiotic peptides that forms part of the defence system of the animal. Here, the antibiotic activity and structure of maculatin 1.1, a 21 amino acid peptide from Litoria genimaculata, are compared. The activity data on maculatin 1.1 and a series of its analogues imply that the mechanism of action of maculatin 1.1 involves binding to, and subsequent lysis of, the bacterial cell membrane. The structure of maculatin 1.1 was determined using NMR spectroscopy in a trifluoroethanol/water mixture and when incorporated into dodecylphosphocholine micelles. Under both conditions, the peptide adopts a very similar conformation, i.e. a helical structure with a central kink in the vicinity of Pro15. The kink allows the peptide to adopt a well-defined amphipathic conformation along its entire length. The similar structures determined under both solvent conditions imply that structures of membrane-interacting peptides in trifluoroethanol/water mixtures are representative of those adopted in a membrane environment, e.g. when incorporated into micelles. The synthetic Ala15 analogue of maculatin 1.1 has markedly reduced activity and its NMR-derived structure is a well-defined helix, which lacks the central kink and flexibility of the parent molecule. It is concluded that the kink is important for full biological activity of the peptide, probably because it allows maximum amphipathicity of the peptide to facilitate interaction with the membrane. The structure of maculatin 1.1 is compared with a related peptide, caerin 1.1 [Wong, H., Bowie, J.H. and Carver, J.A. (1997) Eur. J. Biochem. 247, 545-557], which has an additional central proline residue and enhanced central flexibility compared with maculatin 1.1. The role of central flexibility within antibiotic peptides in their interaction with bacterial membranes is discussed.

Not only the nature of peptide but also the characteristics of cell membrane determine the antimicrobial mechanism of a peptide

The mechanisms of antimicrobial actions of magainin 2, buforin II and poly L-lysine against various Escherichia coli strains were studied. Poly L-lysine inhibited BL21, AD 434 and GroE+/DnaK+ growth without lysing the cell. Magainin 2 had a pore-forming activity on BL 21 and AD 434 membrane but could not inhibit the GroE+/DnaK+ growth in a nutrient-rich medium. Buforin II, which killed BL21 and AD 434 without cell membrane damage, lysed GroE+/DnaK+ to death. Once they were introduced into the cell by electroporation, all three peptides were able to inhibit cell growth at concentrations of 10 times lower than their MICs. These results indicate that the nature of the peptide and also the characteristics of the cell membrane determine the antimicrobial actions of a peptide.

First record of host defence peptides in tadpoles. The magnificent tree frog Litoria splendida

Tadpoles of the Magnificent Tree Frog Litoria splendida produce host defence peptides early in their development and well before metamorphosis. Peptides were identified and characterized using high performance liquid chromatography and electrospray mass spectrometry. No host defence peptides were identified in the eggs. The neuropeptide caerulein was detected 10 d after egg deposition, and the antibiotic peptides caerin 1.1, caerin 1.6 and caerin 3.1 first appeared at 14 d. The concentration of peptides increases with the onset of metamorphosis at 84 d, when the host-defence peptide profile is the same as that of the adult.

The maculatin peptides from the skin glands of the tree frog Litoria genimaculata: a comparison of the structures and antibacterial activities of maculatin 1.1 and caerin 1.1

Six peptides have been isolated and characterized from the dorsal glands of the tree frog Litoria genimaculata. One of these is the known hypotensive peptide caerulein; the others have been named maculatins. The amino acid sequences of the maculatin peptides have been determined using a combination of fast atom bombardment mass spectrometry and automated Edman sequencing. Four of the maculatin peptides show antibiotic activity, with maculatin 1.1 [GLFGVLAKVAAHVVPAIAEHF(NH2)] showing the most pronounced activity, particularly against gram-positive organisms. Maculatin 1.1 resembles the known caerin 1 antibiotic peptides, except that four of the central amino acid residues (of the caerin 1 system) are missing in maculatin 1.1. A comparison of the antibiotic activity of maculatin 1.1 with those of caerin 1.1 is reported.

Mechanism of action of the antimicrobial peptide buforin II: buforin II kills microorganisms by penetrating the cell membrane and inhibiting cellular functions

The mechanism of action of buforin II, which is a 21-amino acid peptide with a potent antimicrobial activity against a broad range of microorganisms, was studied using fluorescein isothiocyanate (FITC)-labeled buforin II and a gel-retardation experiment. Its mechanism of action was compared with that of the well-characterized magainin 2, which has a pore-forming activity on the cell membrane. Buforin II killed Esche-richia coli without lysing the cell membrane even at 5 times minimal inhibitory concentration (MIC) at which buforin II reduced the viable cell numbers by 6 orders of magnitude. However, magainin 2 lysed the cell to death under the same condition. FITC-labeled buforin II was found to penetrate the cell membrane and accumulate inside E. coli even below its MIC, whereas FITC-labeled magainin 2 remained outside or on the cell wall even at its MIC. The gel-retardation experiment showed that buforin II bound to DNA and RNA of the cells over 20 times strongly than magainin 2. All these results indicate that buforin II inhibits the cellular functions by binding to DNA and RNA of cells after penetrating the cell membranes, resulting in the rapid cell death, which is quite different from that of magainin 2 even though they are structurally similar: a linear amphipathic alpha-helical peptide.

New antibiotic caerin 1 peptides from the skin secretion of the Australian tree frog Litoria chloris. Comparison of the activities of the caerin 1 peptides from the genus Litoria

The skin glands of the tree frog Litoria chloris contain a variety of peptides including four antibacterial peptides of the caerin 1 family. Two of these, caerins 1.6 and 1.7, are also present in the related species Litoria xanthomera. The other two peptides, caerins 1.8 and 1.9, are new. Their sequences are: GLFKVLGSVAKHLLPHVVPVIAEKL-NH2 [Caerin 1.8] and GLFGVLGSIAKHVLPHVVPVIAEKL-NH2 [Caerin 1.9]. Comparison of the skin peptide profiles of L. chloris and L. xanthomera confirms that these species are more closely related to each other than to any other species of the genus Litoria that we have studied. A comparison is made of the antibiotic activities of nine members of the caerin 1 family of peptides isolated from tree frogs of the genus Litoria.