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

Assessing the Efficacy of Protease Inactivation for the Preservation of Bioactive Amphibian Skin Peptides

The skin of amphibians is a rich source of peptides with a wide range of biological activities. They are stored in secretory granules in an inactive form. Upon stimulation, they are secreted together with proteases into the skin. Once activated, they rapidly exert their biological effects, including fighting microorganisms and predators, while their excess is immediately destroyed by the released proteases. To keep bioactive peptides in their initial form, it is necessary to inhibit these enzymes. Several inhibitors for this purpose have previously been mentioned; however, there has not been any reliable comparison of their efficiency so far. Here, we studied the efficiency of methanol and hydrochloric and formic acids, as well as phenylmethylsulfonyl fluoride, in the inhibition of nine frog peptides with the known sequence, belonging to five families in the secretion of Pelophylax esculentus. The results demonstrated that methanol had the highest inhibitory efficiency, while phenylmethylsulfonyl fluoride was the least efficient, probably due to its instability in aqueous media. Possible cleavages between certain amino acid residues in the sequence were established for each of the inhibitors. These results may be helpful for future studies on the nature of proteases and on prediction of the possible cleavage sites in novel peptides.

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.

131I-Caerin 1.1 and 131I-Caerin 1.9 for the treatment of non-small-cell lung cancer

Objective

To investigate the effect of the ¹³¹I-labeled high-affinity peptides Caerin 1.1 and Caerin 1.9 for the treatment of A549 human NSCLC cells.

Methods

① 3-[4,5-Dimethylthiazole-2-yl]-2,5-diphenyltetrazolium bromide (MTT) and plate clone formation assays were performed to confirm the in vitro anti-tumor activity of Caerin 1.1 and Caerin 1.9. ② Chloramine-T was used to label Caerin 1.1 and Caerin 1.9 with ¹³¹I, and the Cell Counting Kit 8 assay was performed to analyze the inhibitory effect of unlabeled Caerin 1.1, unlabeled Caerin 1.9, ¹³¹I-labeled Caerin 1.1, and ¹³¹I-labeled Caerin 1.9 on the proliferation of NSCLC cells. An A549 NSCLC nude mouse model was established to investigate the in vivo anti-tumor activity of unlabeled Caerin 1.1, unlabeled Caerin 1.9, ¹³¹I-labeled Caerin 1.1, and ¹³¹I-labeled Caerin 1.9.

Results

① Caerin 1.1 and Caerin 1.9 inhibited the proliferation of NSCLC cells in vitro in a concentration-dependent manner. The half-maximal inhibitory concentration was 16.26 µg/ml and 17.46 µg/ml, respectively, with no significant intergroup difference (P>0.05). ② ¹³¹I-labeled Caerin 1.1 and ¹³¹I-labeled Caerin 1.9 were equally effective and were superior to their unlabeled versions in their ability to inhibit the proliferation and growth of NSCLC cells (P>0.05).

Conclusions

¹³¹I-labeled Caerin 1.1 and ¹³¹I-labeled Caerin 1.9 inhibit the proliferation and growth of NSCLC cells and may become potential treatments for NSCLC.

Caerin 1 Peptides, the Potential Jack-of-All-Trades for the Multiple Antibiotic-Resistant Bacterial Infection Treatment and Cancer Immunotherapy

Antibiotic resistance-related bacterial infections and cancers become huge challenges in human health in the 21^st century. A number of naturally derived antimicrobial peptides possess multiple functions in host defense, including anti-infective and anticancer activities. One of which is known as the caerin 1 family peptides. The microbicidal properties of these peptides have been long discussed. The recent studies also established the usage of two members in this family, caerin 1.1 and caerin 1.9, in antimultiple antibiotic-resistant bacteria species. It is increasingly evident that caerin 1.1 and caerin 1.9 also contain additional activities in the suppression of tumor. In this review, we briefly outline the therapeutic potentials and possible mechanism of action of caerin 1.1 and 1.9 in the treatment of multiple antibiotic-resistant bacterial infection and cancer immunotherapy.

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.

Impact of a Single Point Mutation on the Antimicrobial and Fibrillogenic Properties of Cryptides from Human Apolipoprotein B

Host defense peptides (HDPs) are gaining increasing interest, since they are endowed with multiple activities, are often effective on multidrug resistant bacteria and do not generally lead to the development of resistance phenotypes. Cryptic HDPs have been recently identified in human apolipoprotein B and found to be endowed with a broad-spectrum antimicrobial activity, with anti-biofilm, wound healing and immunomodulatory properties, and with the ability to synergistically act in combination with conventional antibiotics, while being not toxic for eukaryotic cells. Here, a multidisciplinary approach was used, including time killing curves, differential scanning calorimetry, circular dichroism, ThT binding assays, and transmission electron microscopy analyses. The effects of a single point mutation (Pro → Ala in position 7) on the biological properties of ApoB-derived peptide r(P)ApoB_L^Pro have been evaluated. Although the two versions of the peptide share similar antimicrobial and anti-biofilm properties, only r(P)ApoB_L^Ala peptide was found to exert bactericidal effects. Interestingly, antimicrobial activity of both peptide versions appears to be dependent from their interaction with specific components of bacterial surfaces, such as LPS or LTA, which induce peptides to form β-sheet-rich amyloid-like structures. Altogether, obtained data indicate a correlation between ApoB-derived peptides self-assembling state and their antibacterial activity.

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.

Caerin 1 Antimicrobial Peptides That Inhibit HIV and Neisseria May Spare Protective Lactobacilli

Although acquired immunodeficiency syndrome (AIDS) caused by the human immunodeficiency virus (HIV) is a manageable disease for many, it is still a source of significant morbidity and economic hardship for many others. The predominant mode of transmission of HIV/AIDS is sexual intercourse, and measures to reduce transmission are needed. Previously, we showed that caerin 1 antimicrobial peptides (AMPs) originally derived from Australian amphibians inhibited in vitro transmission of HIV at relatively low concentrations and had low toxicity for T cells and an endocervical cell line. The use of AMPs as part of microbicidal formulations would expose the vaginal microbiome to these agents and cause potential harm to protective lactobacilli. Here, we tested the effects of caerin 1 peptides and their analogs on the viability of two species of common vaginal lactobacilli (Lactobacillus rhamnosus and Lactobacillus crispatus). Several candidate peptides had limited toxicity for the lactobacilli at a range of concentrations that would inhibit HIV. Three AMPs were also tested for their ability to inhibit growth of Neisseria lactamica, a close relative of the sexually transmissible Neisseria gonorrhoeae. Neisseria lactamica was significantly more sensitive to the AMPs than the lactobacilli. Thus, several candidate AMPs have the capacity to inhibit HIV and possible N. gonorrhoeae transmission at concentrations that are significantly less harmful to the resident lactobacilli.

Exploring small cationic peptides of different origin as potential antimicrobial agents in aquaculture

Antimicrobial peptides (AMPs) form part of the innate immune response, which is of vital importance in fish, especially in eggs and early larval stages. Compared to antibiotics, AMPs show action against a wider spectrum of pathogens, including viruses, fungi and parasites, are more friendly to the environment, and do not seem to generate resistance in bacteria. Thus, we have tested in vitro the potential use of several synthetic peptides as antimicrobial agents in aquaculture: frog Caerin1.1, European sea bass Dicentracin (Dic) and NK-lysin peptides (NKLPs) and sole NKLP27. Our results demonstrate that the highest bactericidal activity against both human and fish pathogens was obtained with Caerin1.1 followed by sea bass Dic and NKLPs, having the sea bass NKLP20.2 none to negligible activity. Interestingly, Aeromonas salmonicida was refractory to all the fish peptides tested. Regarding the antiviral activity, synthetic peptides were able to inhibit the viral infection of nodavirus (NNV), viral septicaemia haemorrhagic virus (VHSV), infectious pancreatic necrosis virus (IPNV) and spring viremia carp virus (SVCV), which are some of the most devastating virus for aquaculture. However, their effectiveness was highly dependent on the type of virus. Strikingly, IPNV resulted the most resistant virus since Caeerin1.1 and sea bass NKLP20.2 were unable to reduce its titre and the other peptides tested only reduced it to values in the 43-78% range. These data demonstrate that synthetic peptides have great antibacterial and antiviral in vitro activity against important fish pathogens and point to their use as potential therapeutic agents in aquaculture.

Discovery of Novel Caeridins from the Skin Secretion of the Australian White's Tree Frog, Litoria caerulea

Abundant biologically active peptides have been discovered from frog skin secretions, a rich natural source of bioactive compounds with great potential in drug discovery. In this study, three Caeridin peptides, namely, Caeridin-1, S5-Caeridin-1, and Caeridin-a1, were discovered from the skin secretion of the Australian White's tree frog, Litoria caerulea, for the first time, by means of combining transcriptomic and peptidomic analyses. It also represents the first report on bioactive Caeridins since this family of peptides was initially studied 20 years ago. Chemically synthetic versions of each natural Caeridin demonstrated promising bioactivities either on rat smooth muscles or against microbial growth. Specifically, Caeridin-1 produced contraction of rat bladder smooth muscle, while S5-Caeridin-1 induced relaxation of rat ileum smooth muscle, both at nanomolar concentrations. Moreover, Caeridin-a1 was shown to potently inhibit the growth of the planktonic Gram-positive bacteria Staphylococcus aureus (S. aureus), methicillin-resistant S. aureus (MRSA), and Enterococcus faecalis (E. faecalis), the Gram-negative bacterium, Escherichia coli (E. coli), and the yeast, Candida albicans (C. albicans). The discovery of these Caeridins may induce further intensive and systematic studies of frog skin peptides to promote the discovery of natural templates as lead compounds for drug discovery and therapeutic application.

Comparative Proteomic Study of the Antiproliferative Activity of Frog Host-Defence Peptide Caerin 1.9 and Its Additive Effect with Caerin 1.1 on TC-1 Cells Transformed with HPV16 E6 and E7

Caerin is a family of peptides isolated from the glandular secretion of Australian tree frogs, the genus Litoria, and has been previously shown to have anticancer activity against several cancer cells. In this work, we used two host-defence peptides, caerin 1.1 and caerin 1.9, to investigate their ability to inhibit a murine derived TC-1 cell transformed with human papillomavirus 16 E6 and E7 growth in vitro. Caerin 1.9 inhibits TC-1 cell proliferation, although inhibition is more pronounced when applied in conjunction with caerin 1.1. To gain further insights into the antiproliferative mechanisms of caerin 1.9 and its additive effect with caerin 1.1, we used a proteomics strategy to quantitatively examine (i) the changes in the protein profiles of TC-1 cells and (ii) the excretory-secretory products of TC-1 cells following caerin peptides treatment. Caerin 1.9 treatment significantly altered the abundance of several immune-related proteins and related pathways, such as the Tec kinase and ILK signalling pathways, as well as the levels of proinflammatory cytokines and chemokines. In conclusion, caerin peptides inhibit TC-1 cell proliferation, associated with modification in signalling pathways that would change the tumour microenvironment which is normally immune suppressive.

Genital warts treatment: Beyond imiquimod

Genital warts are one of the most common sexually transmitted diseases worldwide. The disease is a result of infection with low-risk types of human papillomaviruses, mostly type 6 and 11. Current therapies for genital warts are mainly ablative, or alternatively topical application of imiquimod cream and sinecatechin (polyphenon E) ointment to the warts. However, low patient compliance and high recurrence rate are significant problems for the treatment of genital warts by imiquimod and ablative therapies. We summarise recent literature in this area and propose combining imiquimod with other therapies to increase the efficacy of imiquimod.

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.

The Amyloid Fibril-Forming Properties of the Amphibian Antimicrobial Peptide Uperin 3.5

The amphibian skin is a vast resource for bioactive peptides, which form the basis of the animals' innate immune system. Key components of the secretions of the cutaneous glands are antimicrobial peptides (AMPs), which exert their cytotoxic effects often as a result of membrane disruption. It is becoming increasingly evident that there is a link between the mechanism of action of AMPs and amyloidogenic peptides and proteins. In this work, we demonstrate that the broad-spectrum amphibian AMP uperin 3.5, which has a random-coil structure in solution but adopts an α-helical structure in membrane-like environments, forms amyloid fibrils rapidly in solution at neutral pH. These fibrils are cytotoxic to model neuronal cells in a similar fashion to those formed by the proteins implicated in neurodegenerative diseases. The addition of small quantities of 2,2,2-trifluoroethanol accelerates fibril formation by uperin 3.5, and is correlated with a structural stabilisation induced by this co-solvent. Uperin 3.5 fibril formation and the associated cellular toxicity are inhibited by the polyphenol (-)-epigallocatechin-3-gallate (EGCG). Furthermore, EGCG rapidly dissociates fully formed uperin 3.5 fibrils. Ion mobility-mass spectrometry reveals that uperin 3.5 adopts various oligomeric states in solution. Combined, these observations imply that the mechanism of membrane permeability by uperin 3.5 is related to its fibril-forming properties.

The membrane-active amphibian peptide caerin 1.8 inhibits fibril formation of amyloid β1-42

The amphibian host-defense peptide caerin 1.8 [(1)GLFKVLGSV(10)AKHLLPHVVP(20)VIAEKL(NH2)] inhibits fibril formation of amyloid β 1-42 [(1)DAEFRHDSG(10)YEVHHQKLVF(20)FAEDVGSNKG(30)AIIGLMVGGV(40)VIA] [Aβ42] (the major precursor of the extracellular fibrillar deposits of Alzheimer's disease). Some truncated forms of caerin 1.8 also inhibit fibril formation of Aβ42. For example, caerin 1.8 (1-13) [(1)GLFKVLGSV(10)AKHL(NH2) and caerin 1.8 (22-25) [KVLGSV(10)AKHLLPHVVP(20)VIAEKL(NH2)] show 85% and 75% respectively of the inhibition activity of the parent caerin 1.8. The synthetic peptide KLVFFKKKKKK is a known inhibitor of Aβ42 fibril formation, and was used as a standard in this study. Caerin 1.8 is the more effective fibril inhibitor. IC50 values (± 15%) are caerin 1.8 (75 μM) and KLVFFKKKKKK (370 μM). MALDI mass spectrometry shows the presence of a small peak corresponding to a protonated 1:1 adduct [caerin 1.8/Aβ42]H(+). Molecular dynamics simulation suggests that both hydrogen bonding and hydrophobic interactions between Aβ42 and caerin 1.8 facilitate the formation of a 1:1 complex in water. Fibril formation from Aβ42 has been proposed to be based around the (16)KLVF(20)F region of Aβ42; this region in the 1:1 complex is partially blocked from attachment of a further molecule of Aβ42.

Polar profile of antiviral peptides from AVPpred Database

Diseases of viral origin in humans are among the most serious threats to health and the global economy. As recent history has shown the virus has a high pandemic potential, among other reasons, due to its ability to spread by air, hence the identification, investigation, containment, and treatment of viral diseases should be considered of paramount importance. In this sense, the bioinformatics research has focused on finding fast and efficient algorithms that can identify highly toxic antiviral peptides and to serve as a first filter, so that trials in the laboratory are substantially reduced. The work presented here contributes to this effort through the use of an algorithm already published by this team, called polarity index method, which identifies with high efficiency antiviral peptides from the exhaustive analysis of the polar profile, using the linear sequence of the peptide. The test carried out included all peptides in APD2 Database and 60 antiviral peptides identified by Kumar and co-workers (Nucleic Acids Res 40:W199-204, 2012), to build its AVPpred algorithm. The validity of the method was focused on its discriminating capacity so we included the 15 sub-classifications of both Databases.

Structural and activity changes in three bioactive anuran peptides when Asp is replaced by isoAsp

The Asp and isoAsp isomers of three bioactive peptides, Crinia angiotensin 11 [APGDRIYHPF(OH)], uperin 1.1 [pEADPNAFYGLM(NH(2))] and citropin 1.1 [GLFDVIKKVASVIGGL(NH(2))] were tested for changes in (i) susceptibility towards proteolytic cleavage, (ii) activity (smooth muscle activity for Crinia angiotensin 11 and uperin 1.1 isomers, and antimicrobial activity for the two isomers of citropin 1.1), and (iii) 3D structures in water, trifluoroethanol-d(3)/water (1:1) and DPC micelles as determined by 2D nuclear magnetic resonance spectroscopy. Proteolytic cleavage with trypsin was identical for each pair of Asp/isoAsp isomers. Cleavage with chymotrypsin was the same for the Crinia angiotensin and uperin 1.1 isomeric pairs, but different for the two Asp/isoAsp citropin 1.1 isomers. Chymotrypsin cleaved at Phe3 (adjacent to Asp4) for citropin 1.1, but not at Phe3 (adjacent to isoAsp4) for isoAsp citropin 1.1. The smooth muscle activity of the isoAsp isomer of Crinia angiotensin 11 was less than that of the Asp isomer. The smooth muscle activity of isoAsp3-uperin 1.1 is greater than that of the Asp isomer at low concentration (<10(-9)M) but no different from the Asp isomer at concentrations>10(-9) M. Citropin 1.1 is a wide-spectrum antibiotic against Gram positive organisms, while the isoAsp isomer is inactive against the test pathogens Staphylococcus aureus and Bacillus subtilis. The observed changes in activity are accompanied by changes in the 3D structures of isomers as determined by 2D nuclear magnetic resonance spectroscopy.

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.

Membrane binding and perturbation studies of the antimicrobial peptides caerin, citropin, and maculatin

Citropin 1.1, maculatin 1.1, and caerin 1.1 are short antibacterial cationic peptides from the skin glands of the Australian tree frog Litoria species. Several analogues have been synthesized to give a better insight into the relationship between the structure of the peptides and their antibacterial and haemolytic activity. Binding studies using a surface plasmon resonance (SPR) biosensor together with a vesicle-capture sensor chip have been used to investigate selectivity of the peptides and their analogues for 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles, as well as for vesicles made from lipid extracts from Escherichia coli and bovine brain. Data obtained for membrane selectivity using natural lipid extracts show better correlation with minimum inhibitory concentration (MIC) values against Gram-positive bacteria and haemolytic activity than that obtained using synthetic DMPG and DMPC. Electron microscopy and membrane leakage studies using Gram-positive bacteria gave further insight into the membrane disruption properties of the peptides. For maculatin 1.1, it was found that the central proline residue, which is responsible for a bend in the alpha-helical structure, is essential not only for the antibacterial activity but also for binding, and perturbation of membranes. The caerin analogues showed only small variations in their MIC values and membrane binding. In contrast, for citropin 1.1, the analogue replacing the aspartate with a lysine showed the lowest MIC against Gram-positive bacteria and best membrane binding to E. coli lipid extracts, coinciding with an increased hydrophobic moment of the peptide. These data give further insight into these antimicrobial natural products, toward the development and evaluation of these and other analogues as potential antibiotics.

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.

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.

Anti-microbial peptides: from invertebrates to vertebrates

Gene-encoded anti-microbial peptides (AMPs) are widespread in nature, as they are synthesized by microorganisms as well as by multicellular organisms from both the vegetal and the animal kingdoms. These naturally occurring AMPs form a first line of host defense against pathogens and are involved in innate immunity. Depending on their tissue distribution, AMPs ensure either a systemic or a local protection of the organism against environmental pathogens. They are classified into three major groups: (i) peptides with an alpha-helical conformation (insect cecropins, magainins, etc.), (ii) cyclic and open-ended cyclic peptides with pairs of cysteine residues (defensins, protegrin, etc.), and (iii) peptides with an over-representation of some amino acids (proline rich, histidine rich, etc.). Most AMPs display hydrophobic and cationic properties, have a molecular mass below 25-30 kDa, and adopt an amphipathic structure (alpha-helix, beta-hairpin-like beta-sheet, beta-sheet, or alpha-helix/beta-sheet mixed structures) that is believed to be essential to their anti-microbial action. Interestingly, in recent years, a series of novel AMPs have been discovered as processed forms of large proteins. Despite the extreme diversity in their primary and secondary structures, all natural AMPs have the in vitro particularity to affect a large number of microorganisms (bacteria, fungi, yeast, virus, etc.) with identical or complementary activity spectra. This review focuses on AMPs forming alpha-helices, beta-hairpin-like beta-sheets, beta-sheets, or alpha-helix/beta-sheet mixed structures from invertebrate and vertebrate origins. These molecules show some promise for therapeutic use.

A protein with antimicrobial activity in the skin of Schlegel’s green tree frog Rhacophorus schlegelii (Rhacophoridae) identified as histone H2B

An extract of the skin of Schlegel's green tree frog, Rhacophorus schlegelii (Anura: Rhacophoridae), contained a protein that inhibited the growth of the Gram-negative bacterium Escherichia coli but was inactive against the Gram-positive bacterium Staphylococcus aureus. The protein was purified to near homogeneity by reverse-phase HPLC and amino acid sequence analysis of the products of an endoproteinase Glu-C digest identified the protein as histone H2B. The complete primary structure of the 125 amino acid residue Rhacophorus histone H2B was determined by nucleotide sequence analysis of a cloned cDNA encoding the protein. Mass spectrometry demonstrated that the protein isolated from the skin was not post-translationally modified. Histone fragments with antimicrobial activity were not identified in the Rhacophorus skin extract nor were cationic, alpha-helical antimicrobial peptides of the kind isolated from the skins of several other frog families. The data provide further evidence that histones play a role in the defense against microorganisms.

Solid-state NMR study of antimicrobial peptides from Australian frogs in phospholipid membranes

Antimicrobial peptides, isolated from the dorsal glands of Australian tree frogs, possess a wide spectrum of biological activity and some are specific to certain pathogens. These peptides have the capability of disrupting bacterial membranes and lysing lipid bilayers. This study focused on the following amphibian peptides: (1) aurein 1.2, a 13-residue peptide; (2) citropin 1.1, with 16 residues; and (3) maculatin 1.1, with 21 residues. The antibiotic activity and structure of these peptides have been studied and compared and possible mechanisms by which the peptides lyse bacterial membrane cells have been proposed. The peptides adopt amphipathic alpha-helical structures in the presence of lipid micelles and vesicles. Specifically 15N-labelled peptides were studied using solid-state NMR to determine their structure and orientation in model lipid bilayers. The effect of these peptides on phospholipid membranes was determined by 2H and 31P solid-state NMR techniques in order to understand the mechanisms by which they exert their biological effects that lead to the disruption of the bacterial cell membrane. Aurein 1.2 and citropin 1.1 are too short to span the membrane bilayer while the longer maculatin 1.1, which may be flexible due to the central proline, would be able to span the bilayer as a transmembrane alpha-helix. All three peptides had a peripheral interaction with phosphatidylcholine bilayers and appear to be located in the aqueous region of the membrane bilayer. It is proposed that these antimicrobial peptides have a "detergent"-like mechanism of membrane lysis.

Nonmammalian vertebrate antibiotic peptides

With the exception of cyclostomes, all vertebrates share the common immune strategy of adaptive, highly specific immunity, based on the products of recombination-activating genes and recombined noninherited receptors for antigens. In addition, they have retained ancient vectors of innate immunity, such as antimicrobial peptides, which are widespread in all eukaryotic organisms and show a high degree of structural homology across most animal taxa. Recently, these substances have become the objects of intensive study for their outstanding bioactive properties with the aim to be applied as very efficient antibiotics, antimicrobials, and even cancerostatics in clinical practice.

Backbone cleavages of [M - H](-) anions derived from caerin 1 peptides and some synthetic modifications. Molecular recognition initiating internal cyclisation of Glu23

The collision induced spectra of [M - H](-) anions from of caerin 1 peptides and some synthetic modifications show the usual alpha, beta and beta' backbone cleavages together with Ser (epsilon,gamma) and Glu (gamma) cleavages which break the peptide backbone in the vicinity of those residues. All of these cleavages require the peptide backbone to be flexible. There is also a backbone cleavage of a type not observed before. This cleavage involves nucleophilic attack of the carboxylate anion of the Glu23 side chain at the backbone CH of Ile 21. We propose that this cleavage requires the caerin peptide to be in an alpha helical conformation (the 3D structure that this peptide adopts in solution) in order that the interacting groups are held in close proximity.

Pseudin-2: an antimicrobial peptide with low hemolytic activity from the skin of the paradoxical frog

Four structurally related peptides (pseudins 1-4) with antimicrobial activity were isolated from an extract of the skin of the paradoxical frog Pseudis paradoxa (Pseudidae). Pseudin-2 (GLNALKKVFQGIHEAIKLINNHVQ) was the most abundant peptide (22 nmol/g tissue) and also the most potent (minimum inhibitory concentrations, MIC = 2.5 microM against Escherichia coli, 80 microM against Staphylococcus aureus, and 130 microM against Candida albicans). The concentration of pseudin-2 producing 50% hemolysis of human erythrocytes was >300 microM. Circular dichroism studies showed that the pseudins belong to the class of cationic, amphipathic alpha-helical antimicrobial peptides but their amino acid sequences are not similar to any previously characterized peptides from frog skin. The pseudins do, however, show sequence similarity with a region at the C-terminus of DEFT, a death effector domain-containing protein expressed in mammalian testicular germ cells that is involved in the regulation of apoptosis.

Antimicrobial peptides from the skin of the Japanese mountain brown frog, Rana ornativentris

Six peptides with antimicrobial activity were isolated from an extract of freeze-dried skin of the Japanese mountain brown frog Rana ornativentris. Two structurally related peptides (brevinin-20a GLFNVFKGALKTAGKHVAGSLLNQLKCKVSGGC, 11 nmol/g dried tissue, and brevinin-20b GIFNVFKGALKTAGKHVAGSLLNQLKCKVSGEC, 170 nmol/g) belong to the brevinin-2 family, previously identified in Asian and European, but not North American, Ranid frogs. Four peptides (temporin-10a FLPLLASLFSRLL.NH2, 13 nmol/g; temporin-10b FLPLIGKILGTI L.NH2, 350 nmol/g; temporin-10c FLPLLASLFSRLF.NH2, 14 nmol/g; and temporin-10d FLPLLASLFSGLF.NH2, 8 nmol/g) are members of the temporin family first identified in the European common frog Rana temporaria but also found in the skins of North American Ranids. The brevinin-2 peptides showed broad-spectrum activity against the gram-positive bacterium, Staphylococcus aureus, the gram-negative bacterium, Escherichia coli and the yeast Candida albicans, whereas the temporins showed potent activity only against S. aureus. The brevinins and temporins belong to the class of cationic antimicrobial peptides that adopt an amphipathic alpha-helical conformation but it is significant that temporin-10d, which lacks a basic amino acid residue, is still active against S. aureus (minimum inhibitory concentration=13 microM compared with 2 microM for temporin-10a). This suggests that strong electrostatic interaction between the peptide and the negatively charged phospholipids of the cell membrane is not an absolute prerequisite for antimicrobial activity.

A protease inhibitor of the Kunitz family from skin secretions of the tomato frog, Dyscophus guineti (Microhylidae)

Norepinephrine-stimulated skin secretions of the tomato frog, Dyscophus guineti, contained a trypsin inhibitor whose primary structure was established as: SPAEVCF LPK(10) ESGLCRARAL(20) RYYYDRGDGK(30) CEEFIYGGCG(40) GNGNNY KSLL(50) TCKISCE. This amino acid sequence identifies the peptide as a member of the Kunitz/bovine pancreatic trypsin inhibitor (BPTI) family and demonstrates that selective evolutionary pressure has acted to conserve those domains in the molecule (corresponding to positions 12-18 and 34-39 in BPTI) that interact with trypsin. Extracellular proteases produced by pathogenic microorganisms play important roles in facilitating invasion of the host and broad spectrum antimicrobial activity of BPTI has been described. Cationic, amphipathic alpha-helical antimicrobial peptides of the magainin type, important in the defense strategy of several species of frog, were not detected in the skin secretions. We speculate, therefore, that synthesis of a proteinase inhibitor in the skin of the tomato frog may be a component of an alternative strategy of this animal to defend itself against microorganisms.

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.

The antibiotic and anticancer active aurein peptides from the Australian Bell Frogs Litoria aurea and Litoria raniformis the solution structure of aurein 1.2

Seventeen aurein peptides are present in the secretion from the granular dorsal glands of the Green and Golden Bell Frog Litoria aurea, and 16 from the corresponding secretion of the related Southern Bell Frog L. raniformis. Ten of these peptides are common to both species. Thirteen of the aurein peptides show wide-spectrum antibiotic and anticancer activity. These peptides are named in three groups (aureins 1-3) according to their sequences. Amongst the more active peptides are aurein 1.2 (GLFDIIKKIAESF-NH2), aurein 2.2 (GLFDIVKKVVGALGSL-NH2) and aurein 3.1 (GLFDIVKKIAGHIAGSI-NH2). Both L. aurea and L. raniformis have endoproteases that deactivate the major membrane-active aurein peptides by removing residues from both the N- and C-termini of the peptides. The most abundant degradation products have two residues missing from the N-terminal end of the peptide. The solution structure of the basic peptide, aurein 1.2, has been determined by NMR spectroscopy to be an amphipathic alpha-helix with well-defined hydrophilic and hydrophobic regions. Certain of the aurein peptides (e.g. aureins 1.2 and 3.1) show anticancer activity in the NCI test regime, with LC50 values in the 10-5-10-4 M range. The aurein 1 peptides have only 13 amino-acid residues: these are the smallest antibiotic and anticancer active peptides yet reported from an anuran. The longer aurein 4 and 5 peptides, e.g. aurein 4.1 (GLIQTIKEKLKELAGGLVTGIQS-OH) and aurein 5. 1 (GLLDIVTGLLGNLIVDVLKPKTPAS-OH) show neither antibacterial nor anticancer activity.

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.

Kassinatuerin-1: a peptide with broad-spectrum antimicrobial activity isolated from the skin of the hyperoliid frog, Kassina senegalensis

Kassinatuerin-1 (GFMKYIGPLI(10)PHAVKAISDL(20)I.NH(2)) was isolated in high yield (75 nmol/g) from an extract of the skin of a Hyperoliid frog, the African running frog Kassina senegalensis and its sequence was confirmed by total synthesis. The peptide inhibited growth of the gram-negative bacterium Escherichia coli (minimum inhibitory concentration, MIC = 4 microM), the gram-positive bacterium Staphylococcus aureus (MIC = 8 microM), and the yeast Candida albicans (MIC = 70 microM). A structurally related peptide, kassinatuerin-2 (FIQYLAPLI(10)PHAVKAISDL(20)I.NH(2)) was also isolated in high yield (96 nmol/g) from the extract but was devoid of antimicrobial activity against these microrganisms. Kassinatuerin-1 may be classified with other linear, cationic antimicrobial peptides that can potentially adopt an amphipathic alpha-helical conformation but it contains almost no amino acid sequence identity with previously characterized bioactive peptides from frog skin.

Peptides with antimicrobial activity from four different families isolated from the skins of the North American frogs Rana luteiventris, Rana berlandieri and Rana pipiens

The skins of frogs of the genus Rana synthesize a complex array of antimicrobial peptides that may be grouped into eight families on the basis of structural similarity. A total of 24 peptides with differential growth-inhibitory activity towards the Gram-positive bacterium Staphylococcus aureus, the Gram-negative bacterium Escherichia coli and the yeast Candida albicans were isolated from extracts of the skins of three closely related North American frogs, Rana luteiventris (spotted frog), Rana berlandieri (Rio Grande leopard frog) and Rana pipiens (Northern leopard frog). Structural characterization of the antimicrobial peptides demonstrated that they belonged to four of the known families: the brevinin-1 family, first identified in skin of the Asian frog Rana porosa brevipoda; the esculentin-2 family, first identified in the European frog Rana esculenta; the ranatuerin-2 family, first identified in the North American bullfrog Rana catesbeiana; and the temporin family, first identified in the European frog Rana temporaria. Peptides belonging to the brevinin-2, ranalexin, esculentin-1 and ranatuerin-1 families were not identified in the extracts. Despite the close phylogenetic relationship between the various species of Ranid frogs, the distribution and amino-acid sequences of the antimicrobial peptides produced by each species are highly variable and species-specific, suggesting that they may be valuable in taxonomic classification and molecular phylogenetic analysis.

Differences in the skin peptides of the male and female Australian tree frog Litoria splendida. The discovery of the aquatic male sex pheromone splendipherin, together with phe8 caerulein and a new antibiotic peptide caerin 1.10

The skin secretions of female and male Litoria splendida have been monitored monthly over a three-year period using HPLC and electrospray mass spectrometry. Two minor peptides are present only in the skin secretion of the male. The first of these is the female-attracting aquatic male sex pheromone that we have named splendipherin, a 25 amino acid peptide (GLVSSIGKALGGLLADVVKSKGQPA-OH). This pheromone constitutes about 1% of the total skin peptides during the breeding season (January to March), dropping to about 0.1% during the period June to November. Splendipherin attracts the female in water at a concentration of 10-11-10-9 M, and is species specific. The second peptide is a wide-spectrum antibiotic of the caerin 1 group, a 25 residue peptide (GLLSVLGSVAKHVLPHVVPVIAEKL-NH2) named caerin 1.10. The neuropeptides of L. splendida are also seasonally variable, the change identical for both the female and male. During the period October to March, the sole neuropeptide present in skin secretions is caerulein [pEQDY(SO3)TGWMDF-NH2]; this is active on smooth muscle and is also an analgaesic. During the southern winter (June to September), more than half of the caerulein is hydrolysed to [pEQDYTGWMDF-NH2], a peptide that shows no smooth muscle activity. In place of caerulein, a new peptide, Phe8 caerulein [pEQDY(SO3)TGWFDF-NH2], becomes a major component of the skin secretion. Perhaps this seasonal change is involved in thermoregulation, that is, with the initiation and maintenance of the inactive (hibernation) phase of the animal.

Peptides with antimicrobial activity of the brevinin-1 family isolated from skin secretions of the southern leopard frog, Rana sphenocephala

Three peptides with growth-inhibitory activity towards the gram-negative bacterium Eschericia coli were isolated from electrically stimulated secretions from the skin of the southern leopard frog, Rana sphenocephala. Structural characterization demonstrated that the peptides [brevinin-1Sa, minimum inhibitory concentration (MIC) = 55 microM; brevinin-1Sb, MIC = 17 microM; brevinin-1Sc, MIC = 14 microM] represent new members of the brevinin-1 family of antimicrobial peptides, previously isolated from several other species of frogs of the genus Rana. Their high concentration in skin secretions and extreme variability in amino acid sequence suggest that the brevinin family of peptides may be of value as molecular markers for the identification and taxonomic classification of Ranid frogs.

Host defence peptides from the skin glands of the Australian blue mountains tree-frog Litoria citropa. Solution structure of the antibacterial peptide citropin 1.1

Nineteen citropin peptides are present in the secretion from the granular dorsal glands of the Blue Mountains tree-frog Litoria citropa; 15 of these peptides are also present in the secretion from the submental gland. Two major peptides, citropin 1.1 (GLFDVIKKVASVIGGL-NH2), citropin 1.2 (GLFDIIKKVASVVGGL-NH2) and a minor peptide, citropin 1.3 (GLFDIIKKVASVIGGL-NH2) are wide-spectrum antibacterial peptides. The amphibian has an endoprotease which deactivates these membrane-active peptides by removing residues from the N-terminal end: loss of three residues gives the most abundant degradation products. The solution structure of the basic peptide citropin 1.1 has been determined by NMR spectroscopy [in a solvent mixture of trifluoroethanol/water (1 : 1)] to be an amphipathic alpha-helix with well-defined hydrophobic and hydrophilic regions. The additional four peptides produced by the dorsal glands are structurally related to the antibacterial citropin 1 peptides but contain three more residues at their C-terminus [e.g. citropin 1.1.3 (GLFDVIKKVASVIGLASP-OH)]. These peptides show minimal antibacterial activity; their role in the amphibian skin is not known.

The solution structure of uperin 3.6, an antibiotic peptide from the granular dorsal glands of the Australian toadlet, Uperoleia mjobergii

Uperin 3.6 (GVIDA5AKKVV10NVLKN15LF-NH2) is a wide-spectrum antibiotic peptide isolated from the Australian toadlet, Uperoleia mjobergii. With only 17 amino acid residues, it is smaller than most other wide-spectrum antibiotic peptides isolated from amphibians. In 50% (by vol.) trifluoroethanol, an NMR study and structure calculations indicate that uperin 3.6 adopts a well-defined amphipathic alpha-helix with distinct hydrophilic and hydrophobic faces. Examination of the activities of synthetic modifications of uperin 3.6 reveal that the three lysine residues are essential for antibiotic activity.

Ranatuerin 1T: an antimicrobial peptide isolated from the skin of the frog, Rana temporaria

A peptide, termed ranatuerin 1T, with growth-inhibiting activity toward Staphylococcus aureus, was isolated from an extract of the skin of the European brown frog, Rana temporaria. The primary structure of the peptide was established as: GLLSGLKKVG10 KHVAKNVAVS20LMDSLKCKIS30GDC. In common with other anti-microbial peptides from Ranid frogs, (e.g., ranalexin, ranatuerins, gaegurins, brevinins, esculetins, rugosins), ranatuerin IT contains an intramolecular disulfide bridge forming a heptapeptide ring but there is little structural similarity outside this cyclic region. The minimum inhibitory concentration (MIC) of ranatuerin 1T was 120 microM against the Gram-positive bacterium S. aureus and 40 microM against the Gram-negative bacterium Escherichia coli, but the peptide was not active against the yeast Candida albicans.

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.