Purification and characterization of antimicrobial and vasorelaxant peptides from skin extracts and skin secretions of the North American pig frog Rana grylio

A Comprehensive Review of Patented Antimicrobial Peptides from Amphibian Anurans

Since the 1980s, studies of antimicrobial peptides (AMPs) derived from anuran skin secretions have unveiled remarkable structural diversity and a wide range of activities. This study explores the potential of these peptides for drug development by examining granted patents, amino acid modifications related to patented peptides, and recent amphibians' taxonomic updates influencing AMP names. A total of 188 granted patents related to different anuran peptides were found, with Asia and North America being the predominant regions, contributing 65.4% and 15.4%, respectively. Conversely, although the Neotropical region is the world's most diversified region for amphibians, it holds only 3.7% of the identified patents. The antimicrobial activities of the peptides are claimed in 118 of these 188 patents. Additionally, for 160 of these peptides, 66 patents were registered for the natural sequence, 69 for both natural and derivative sequences, and 20 exclusively for sequence derivatives. Notably, common modifications include alterations in the side chains of amino acids and modifications to the peptides' N- and C-termini. This review underscores the biomedical potential of anuran-derived AMPs, emphasizing the need to bridge the gap between AMP description and practical drug development while highlighting the urgency of biodiversity conservation to facilitate biomedical discoveries.

Antimicrobial peptides for combating drug-resistant bacterial infections

The problem of drug resistance due to long-term use of antibiotics has been a concern for years. As this problem grows worse, infections caused by multiple bacteria are expanding rapidly and are extremely detrimental to human health. Antimicrobial peptides (AMPs) are a good alternative to current antimicrobials with potent antimicrobial activity and unique antimicrobial mechanisms, which have advantages over traditional antibiotics in fighting against drug-resistant bacterial infections. Currently, researchers have conducted clinical investigations on AMPs for drug-resistant bacterial infections while integrating new technologies in the development of AMPs, such as changing amino acid structure of AMPs and using different delivery methods for AMPs. This article introduces the basic properties of AMPs, deliberates the mechanism of drug resistance in bacteria and the therapeutic mechanism of AMPs. The current disadvantages and advances of AMPs in combating drug-resistant bacterial infections are also discussed. This article provides important insights into the research and clinical application of new AMPs for drug-resistant bacterial infections.

Peptides Isolated from Amphibian Skin Secretions with Emphasis on Antimicrobial Peptides

The skin of amphibians is a tissue with biological functions, such as defense, respiration, and excretion. In recent years, researchers have discovered a large number of peptides in the skin secretions of amphibians, including antimicrobial peptides, antioxidant peptides, bradykinins, insulin-releasing peptides, and other peptides. This review focuses on the origin, primary structure, secondary structure, length, and functions of peptides secreted from amphibians' skin. We hope that this review will provide further information and promote the further study of amphibian skin secretions, in order to provide reference for expanding the research and application of amphibian bioactive peptides.

Overall assessment of antibiotic substitutes for pigs: a set of meta-analyses

Background

Antibiotic growth promoters are widely used to improve weight gain. However, the abuse of antibiotics can have many negative effects on people. Developing alternatives to antibiotics is an urgent need in livestock production. We aimed to perform a meta-analysis and network meta-analysis (NMA) to investigate the effects of feed additives as potential antibiotic substitutes (ASs) on bacteriostasis, growth performance, intestinal morphology and immunity. Furthermore, the primary, secondary, and tertiary ASs were defined by comparing their results with the results of antibiotics.

Results

Among 16,309 identified studies, 37 were summarized to study the bacteriostasis effects of feed additives, and 89 were included in the meta-analysis and NMA (10,228 pigs). We summarized 268 associations of 57 interventions with 32 bacteria. The order of bacteriostasis effects was as follows: antimicrobial peptides (AMPs) ≈ antibiotics>organic acids>plant extracts>oligosaccharides. We detected associations of 11 feed additives and 11 outcomes. Compared with a basal diet, plant extract, AMPs, probiotics, microelements, organic acids, bacteriophages, lysozyme, zymin, and oligosaccharides significantly improved growth performance (P < 0.05); organic acids, probiotics, microelements, lysozyme, and AMPs remarkably increased the villus height:crypt depth ratio (V/C) (P < 0.05); and plant extracts, zymin, microelements, probiotics, and organic acids notably improved immunity (P < 0.05). The optimal AMP, bacteriophage, lysozyme, microelements, oligosaccharides, organic acids, plants, plant extracts, probiotics, and zymin doses were 0.100%, 0.150%, 0.012%, 0.010%, 0.050%, 0.750%, 0.20%, 0.040%, 0.180%, and 0.100%, respectively. Compared with antibiotics, all investigated feed additives exhibited no significant difference in effects on growth performance, IgG, and diarrhoea index/rate (P > 0.05); AMPs and microelements significantly increased V/C (P < 0.05); and zymin significantly improved lymphocyte levels (P < 0.05). Furthermore, linear weighting sum models were used to comprehensively estimate the overall impact of each feed additive on pig growth and health.

Conclusions

Our findings suggest that AMPs and plant extracts can be used as primary ASs for weaned piglets and growing pigs, respectively. Bacteriophages, zymin, plants, probiotics, oligosaccharides, lysozyme, and microelements can be regarded as secondary ASs. Nucleotides and organic acids can be considered as tertiary ASs. Future studies should further assess the alternative effects of combinational feed additives.

Antimicrobial Peptides: Amphibian Host Defense Peptides

Antimicrobial Peptides (AMPs) are one of the most common components of the innate immune system that protect multicellular organisms against microbial invasion. The vast majority of AMPs are isolated from the frog skin. Anuran (frogs and toads) skin contains abundant AMPs that can be developed therapeutically. Such peptides are a unique but diverse group of molecules. In general, more than 50% of the amino acid residues form the hydrophobic part of the molecule. Normally, there are no conserved structural motifs responsible for activity, although the vast majority of the AMPs are cationic due to the presence of multiple lysine residues; this cationicity has a close relationship with antibacterial activity. Notably, recent evidence suggests that synthesis of AMPs in frog skin may confer an advantage on a particular species, although they are not essential for survival. Frog skin AMPs exert potent activity against antibiotic-resistant bacteria, protozoa, yeasts, and fungi by permeating and destroying the plasma membrane and inactivating intracellular targets. Importantly, since they do not bind to a specific receptor, AMPs are less likely to induce resistance mechanisms. Currently, the best known amphibian AMPs are esculentins, brevinins, ranacyclins, ranatuerins, nigrocin-2, magainins, dermaseptins, bombinins, temporins, and japonicins-1 and -2, and palustrin-2. This review focuses on these frog skin AMPs and the mechanisms underlying their antimicrobial activity. We hope that this review will provide further information that will facilitate further study of AMPs and cast new light on novel and safer microbicides.

Temporins: An Approach of Potential Pharmaceutic Candidates

Antimicrobial peptides (AMPs), also known as host defense peptides, are small and mostly polycationic molecules that form part of the innate immune response. There are currently more than 3000 experimentally reported AMPs. Particularly in frogs, the temporin family has been discovered as potential AMPs. The aim of this work is to review the latest publications about this class of peptides, discuss their properties, and present an update of the last studies and new discoveries in the field. More than 130 temporins have been identified in this family. The most studied temporins are temporin A (TA), temporin B (TB), and temporin L (TL). These peptides showed antimicrobial activity against gram-negative, gram-positive bacteria and fungi. Since the discovery of temporins in 1996, several groups of researchers isolated different peptides from various species of frogs that were included as members of this family. Although antimicrobial activity of many temporins has not been analyzed yet, most of them showed antimicrobial and antifungal activities. A combination of nanotechnology and AMPs for temporins in different antimicrobial treatments could be a promising alternative for resistant pathogens. These studies demonstrate that, even with the advancement in scientific research on the composition and antimicrobial activity of temporins, further studies are necessary to wholly understand their components and mechanisms of action.

Antibacterial, antifungal, anticancer activities and structural bioinformatics analysis of six naturally occurring temporins

Antimicrobial peptides are a special class of natural products with potential applications as novel therapeutics. This study focuses on six temporins (four with no activity data and two as positive controls). Using synthetic peptides, we report antibacterial, antifungal, and anticancer activities of temporins-CPa, CPb, 1Ga, 1Oc, 1Ola, and 1SPa. While temporin-1Ga and temporin-1OLa showed higher antifungal and anticancer activity, most of these peptides were active primarily against Gram-positive bacteria. Temporin-1OLa, with the highest cell selectivity index, could preferentially kill methicillin-resistant Staphylococcus aureus (MRSA), consistent with a reduced hemolysis in the presence of bacteria. Mechanistically, temporin-1OLa rapidly killed MRSA by damaging bacterial membranes. Using micelles as a membrane-mimetic model, we determined the three-dimensional structure of temporin-1OLa by NMR spectroscopy. The peptide adopted a two-domain structure where a hydrophobic patch is followed by a classic amphipathic helix covering residues P3-I12. Such a structure is responsible for anti-biofilm ability in vitro and in vivo protection of wax moths Galleria mellonella from staphylococcal infection. Finally, our bioinformatic analysis leads to a classification of temporins into six types and confers significance to this NMR structure since temporin-1OLa shares a sequence model with 62% of temporins. Collectively, our results indicate the potential of temporin-1OLa as a new anti-MRSA compound, which shows an even better anti-biofilm capability in combination with linezolid.

Cytotoxic peptides with insulin-releasing activities from skin secretions of the Italian stream frog Rana italica (Ranidae)

Peptidomic analysis of norepinephrine-stimulated skin secretions from Italian stream frog Rana italica led to the purification and characterization of two host-defense peptides differing by a single amino acid residue belonging to the brevinin-1 family (brevinin-1ITa and -1ITb), a peptide belonging to the temporin family (temporin-ITa) and a component identified as prokineticin Bv8. The secretions contained relatively high concentrations of the methionine-sulphoxide forms of brevinin-1ITa and -1ITb suggesting that these peptides may have a role as antioxidants in the skin of this montane frog. Brevinin-1ITa (IVPFLLGMVPKLVCLITKKC) displayed potent cytotoxicity against non-small cell lung adenocarcinoma A549 cells (LC₅₀  = 18 μM), breast adenocarcinoma MDA-MB-231 cells (LC₅₀  = 8 μM) and colorectal adenocarcinoma HT-29 cells (LC₅₀  = 18 μM), but the peptide was also strongly hemolytic against mouse erythrocytes (LC₅₀  = 7 μM). Temporin-ITa (VFLGAIAQALTSLLGKL.NH₂ ) was between three and fivefold less potent against these cells. Brevinin-1ITa inhibited growth of both Gram-positive Staphylococcus epidermidis and Gram-negative Escherichia coli as well as a strain of the opportunist yeast pathogen Candida parapsilosis, whereas temporin-ITa was active only against S. epidermidis and C. parapsilosis. Both peptides stimulated the release of insulin from BRIN-BD11 clonal β-cells at concentrations ≥1 nM, but brevinin-1ITa was cytotoxic to the cells at concentrations ≥3 μM. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

The diversity and evolution of anuran skin peptides

Amphibians exhibit various, characteristic adaptations related to their "incomplete" shift from the aquatic to the terrestrial habitat. In particular, the integument was subject to a number of specialized modifications during the evolution of these animals. In this review, we place special emphasis on endogenous host-defence skin peptides from the cuteanous granular glands anuran amphibians (frogs and toads). The overview on the two broad groups of neuroactive and antimicrobial peptides (AMPs) goes beyond a simple itemization in that we provide a new perspective into the evolution and function of anuran AMPs. Briefly, these cationic, amphipathic and α-helical peptides are traditionally viewed as being part of the innate immune system, protecting the moist skin against invading microorganisms through their cytolytic action. However, the complete record of anuran species investigated to date suggests that AMPs are distributed sporadically (i.e., non-universally) across Anura. Together with the intriguing observation that virtually all anurans known to produce neuropeptides in their granular glands also co-secrete cytolytic peptides, we call the traditional role for AMPs as being purely antimicrobial into question and present an alternative scenario. We hypothesize AMPs to assist neuroactive peptides in their antipredator role through their cytolytic action increasing the delivery of the latter to the endocrine and nervous system of the predator. Thus, AMPs are more accurately viewed as cytolysins and their contribution to the immune system is better regarded as an accessory benefit.

Short native antimicrobial peptides and engineered ultrashort lipopeptides: similarities and differences in cell specificities and modes of action

Due to the rapid emergence of resistant microbes to the currently available antibiotics, cationic antimicrobial peptides have attracted considerable interest as a possible new generation of anti-infective compounds. However, low cost development for therapeutic or industrial purposes requires, among other properties, that the peptides will be small and with simple structure. Therefore, considerable research has been devoted to optimizing peptide length combined with a simple design. This review focuses on the similarities and differences in the mode of action and target cell specificity of two families of small peptides: the naturally occurring temporins from the skin of amphibia and the engineered ultrashort lipopeptides. We will also discuss the finding that acylation of cationic peptides results in molecules with a more potent spectrum of activity and a higher resistance to proteolytic degradation. Conjugation of fatty acids to linear native peptide sequences is a powerful strategy to engineer novel successful anti-infective drugs.

Antimicrobial peptides from amphibians

Increased prevalence of multi-drug resistance in pathogens has encouraged researchers to focus on finding novel forms of anti-infective agents. Antimicrobial peptides (AMPs) found in animal secretions are components of host innate immune response and have survived eons of pathogen evolution. Thus, they are likely to be active against pathogens and even those that are resistant to conventional drugs. Many peptides have been isolated and shown to be effective against multi-drug resistant pathogens. More than 500 AMPs have been identified from amphibians. The abundance of AMPs in frog skin is remarkable and constitutes a rich source for design of novel pharmaceutical molecules. Expression and post-translational modifications, discovery, activities and probable therapeutic application prospects of amphibian AMPs will be discussed in this article.

Peptidomic analysis of skin secretions from the bullfrog Lithobates catesbeianus (Ranidae) identifies multiple peptides with potent insulin-releasing activity

Using a combination of reversed-phase HPLC and electrospray mass spectrometry, peptidomic analysis of norepinephrine-stimulated skin secretions of the American bullfrog Lithobates catesbeianus Shaw, 1802 led to the identification and characterization of five newly described peptides (ranatuerin-1CBb, ranatuerin-2CBc, and -CBd, palustrin-2CBa, and temporin-CBf) together with seven peptides previously isolated on the basis of their antimicrobial activity (ranatuerin-1CBa, ranatuerin-2CBa, brevinin-1CBa, and -1CBb, temporin-CBa, -CBb, and -CBd). The abilities of the most abundant of the purified peptides to stimulate the release of insulin from the rat BRIN-BD11 clonal β cell line were evaluated. Ranatuerin-2CBd (GFLDIIKNLGKTFAGHMLDKIRCTIGTCPPSP) was the most potent peptide producing a significant stimulation of insulin release (119% of basal rate, P<0.01) from BRIN-BD11 cells at a concentration of 30nM, with a maximum response (236% of basal rate, P<0.001) at a concentration of 3μM. Ranatuerin-2CBd did not stimulate release of the cytosolic enzyme, lactate dehydrogenase at concentrations up to 3μM, indicating that the integrity of the plasma membrane had been preserved. Brevinin-1CBb (FLPFIARLAAKVFPSIICSVTKKC) produced the maximum stimulation of insulin release (285% of basal rate, P<0.001 at 3μM) but the peptide was cytotoxic at this concentration.

The contribution of skin antimicrobial peptides to the system of innate immunity in anurans

Cationic peptides with the propensity to adopt an amphipathic α-helical conformation in a membrane-mimetic environment are synthesized in the skins of many species of anurans (frogs and toads). These peptides frequently display cytolytic activities against a range of pathogenic bacteria and fungi consistent with the idea that they play a role in the host's system of innate immunity. However, the importance of the peptides in the survival strategy of the animal is not clearly understood. It is a common misconception that antimicrobial peptides are synthesized in the skins of all anurans. In fact, the species distribution is sporadic suggesting that their production may confer some evolutionary advantage to the organism but is not necessary for survival. Although growth inhibitory activity against the chytrid fungus Batrachochytrium dendrobatidis, responsible for anuran population declines worldwide, has been demonstrated in vitro, the ability of frog skin antimicrobial peptides to protect the animal in the wild appears to be limited and there is no clear correlation between their production by a species and its resistance to fatal chytridiomycosis. The low potency of many frog skin antimicrobial peptides is consistent with the hypothesis that cutaneous symbiotic bacteria may provide the major system of defense against pathogenic microorganisms in the environment with antimicrobial peptides assuming a supplementary role in some species.

[Molecular diversities and functions of antibacterial peptides from the skins of Ranidae of amphibians.]

Granular glands in the frog skins of Ranidae of amphibians, a widely distributed group with over 650 species, synthesize and secrete a remarkably diverse array of peptides with the broad-spectrum antibacterial, antifungal and other biologic activities to protect the organism against a wide range of pathogens, which are believed to have arisen as a result of multiple gene duplication events. Almost without exception, these components are hydrophobic, cationic and form an amphipathic a-helix in a membrane-mimetic solvent. The peptides can be grouped into families on the basis of structural similarity. To date, brevinin-1, esculentin-1, esculentin-2, and temporin peptides, ranalexin, ranatuerin-1, ranatuerin-2 and palustrin, brevinin-2, tigerinin, japonicin, nigrocin and melittin-related peptides have been found in amphibians of Ranidae. In this paper, the molecular diversity, structural feature and the biological ac-tivity of Ranidae antibacterial peptides were reviewed.

Biological characterization and modes of action of temporins and bombinins H, multiple forms of short and mildly cationic anti-microbial peptides from amphibian skin

Genetically encoded cationic anti-microbial peptides (AMPs) are essential components of the ancient and non-specific innate immune system, which is the principal defence mechanism of all species of life, with the primary role to kill infectious microorganisms. Amphibian skin is one of the richest natural sources of such molecules, which are produced by holocrine-type dermal glands and released upon stimulation. This review highlights the attractive and unique structural/functional properties of temporins and bombinins H, two families of short and mildly cationic peptides, isolated from the skin of frogs belonging to Rana and Bombina genera, respectively. Beside improving our knowledge on the role of AMPs in the regulation of the innate immunity, the biological significance of the existence of multiple forms of a prototypic peptide sequence within the same organism and the implication of short peptides in the endotoxin neutralization, these two classes of AMPs can be also considered as valid candidates for the design of novel anti-infective and anti-sepsis drugs.

Fluorescent temporin B derivative and its binding to liposomes

Temporins are short (10-13 amino acids) and linear antimicrobial peptides first isolated from the skin of the European red frog, Rana temporaria, and are effective against Gram-positive bacteria and Candida albicans. Similarly to other antimicrobial peptides, the association of temporins to lipid membranes has been concluded to underlie their antimicrobial effects. Accordingly, a detailed understanding of their interactions with phospholipids is needed. We conjugated a fluorophore (Texas Red) to a Cys containing derivative of temporin B (temB) and investigated its binding to liposomes by fluorescence spectroscopy. Circular dichroic spectra for the Cys-mutant recorded in the absence and in the presence of phospholipids were essentially similar to those for temB. A blue shift in the emission spectra and diminished quenching by ferrocyanide (FCN) of Texas Red labeled temporin B (TRC-temB) were seen in the presence of liposomes. Both of these changes can be attributed to the insertion of the Texas Red into the hydrophobic region of the bilayer. Resonance energy transfer, steady state anisotropy, and fluorescence lifetimes further demonstrate the interaction of TRC-temB with liposomes to be enhanced by negatively charged phospholipids. Instead, cholesterol attenuates the association of TRC-temB with membranes. The interactions between TRC-temB and liposomes of varying negative surface charge are driven by electrostatics as well as hydrophobicity. Similarly to native temporin B also TRC-temB forms amyloid type fibers in the presence of negatively charged liposomes. This property is likely to relate to the cytotoxic activity of this peptide.

Peptidomic analysis of skin secretions from Rana heckscheri and Rana okaloosae provides insight into phylogenetic relationships among frogs of the Aquarana species group

The members of the Aquarana (or Rana catesbeiana species group) form a monophyletic group comprising seven species: R. catesbeiana, Rana clamitans, Rana grylio, Rana virgatipes, Rana septentrionalis, Rana heckscheri and Rana okaloosae. Previous work has led to structural characterization of the antimicrobial peptides present in electrically-stimulated skin secretions from the first five species listed and this study presents the primary structures of orthologs from the river frog R. heckscheri and the Florida bog frog R. okaloosae. Peptidomic analysis of R. heckscheri and R. okaloosae skin secretions led to the identification of peptides with antimicrobial activity belonging to the ranalexin, ranatuerin-2, and temporin families. In addition, a peptide (GFLDIIKDTGKDFAVKILNNLKCKLAGGCPR) was isolated from R. okaloosae whose primary structure identified it as a member of the palustrin-2 family. Consistent with previous data based upon morphological analysis and comparisons of the nucleotide sequences of mitochondrial and ribosomal genes, cladistic analysis based upon a comparison of the amino acid sequences of antimicrobial peptides indicates a sister-group relationship between R. heckscheri and R. grylio and a close, but less well defined, phylogenetic relationship between R. okaloosae and R. clamitans.

Purification and characterization of antimicrobial peptides from the skin secretions of the carpenter frog Rana virgatipes (Ranidae, Aquarana)

The members of the Aquarana (or Rana catesbeiana species group) form a well-supported monophyletic clade but phylogenetic relationships between species within the group are incompletely understood. Peptides that differentially inhibited the growth of bacteria were purified from electrically stimulated skin secretions of the carpenter frog Rana virgatipes. Structural characterization identified members of the ranatuerin-2 (3 peptides) and temporin (3-peptides) families, previously found in the skins of R. catesbeiana, R. clamitans, R. grylio and R. septentrionalis. Ranalexin, a peptide previously found only in the Aquarana, was isolated together with a variant (FFGLHNLVPSMLCVVRKKC) that lacks the propensity to adopt an alpha-helical conformation and so was devoid of antimicrobial activity. Two C-terminally alpha-amidated peptides belonging to the brevinin-2 family were isolated from the skin secretions that, like an ortholog from R. septentrionalis, lacked the C-terminal cyclic heptapeptide domain associated with members of this family. Ranatuerin-1, previously isolated from R. catesbeiana, R. clamitans and R. grylio but absent from R. septentrionalis, was also not identified in R. virgatipes. Synthetic replicates of temporin-1Va (FLSSIGKILGNLL.NH2), temporin-IVb (FLSIIAKVLGSLF.NH2) and temporin-1Vc (FLPLVTMLLGKLF.NH2) potently inhibited growth of Gram-positive bacteria (including methicillin-resistant Staphylococcus aureus). Temporin-1Va was also active against Gram-negative bacteria and the opportunistic yeast pathogen Candida albicans and had relatively weak hemolytic activity (LD50=120 microM) and may therefore represent a candidate for drug development. Our data support the placement of R. virgatipes in the Aquarana and indicate a closer phylogenetic relationship of R. virgatipes with R. septentrionalis than with R. catesbeiana, R. clamitans and R. grylio.

An antimicrobial peptide from the skin secretions of the mountain chicken frog Leptodactylus fallax (Anura:Leptodactylidae)

A 25 amino-acid-residue, C-terminally alpha-amidated peptide with antimicrobial activity, which has been termed fallaxin, was isolated in high yield from the norepinephrine-stimulated skin secretions of the mountain chicken frog Leptodactylus fallax (Anura:Leptodactylidae). The amino acid sequence of the peptide (Gly-Val-Val-Asp-Ile-Leu-Lys-Gly-Ala-Ala-Lys-Asp-Ile-Ala-Gly-His-Leu-Ala-Ser-Lys-Val-Met-Asn-Lys-Leu.NH2) shows structural similarity with members of the ranatuerin-2 family previously isolated from the skins of frogs of the genus Rana that are only distantly related to the Leptodactylidae. This observation is consistent with the hypothesis that many frog skin antimicrobial peptides are related evolutionarily, having arisen from multiple duplications of an ancestral gene that existed before the radiation of the different families. Fallaxin inhibited the growth of reference strains of Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa, Enterobacter cloacae, Klebsiella pneumoniae) but with relatively low potency (MIC> or =20 microM) and was inactive against the Gram-positive bacterium (Staphylococcus aureus) and the yeast Candida albicans. The hemolytic activity of fallaxin was very low (HC50>200 microM). A second peptide, comprising residues (1-22) of fallaxin, was also isolated from the skin secretions but this component was inactive against the microorganisms tested.

Peptide Tyrosine Arginine, a potent immunomodulatory peptide isolated and structurally characterized from the skin secretions of the dusky gopher frog, Rana sevosa

An octadecapeptide was isolated from the skin secretions of the dusky gopher frog (Rana sevosa) on the basis of histamine release from rat peritoneal mast cells. This peptide was purified to homogeneity by HPLC and found to have the following primary structure, YLKGCWTKSYPPKPCFSR, using both Edman degradation chemistry and peptide sequencing using high-resolution mass spectrometry (Q-TOF MS). The peptide, named peptide Tyrosine Arginine (pYR) shares 77.8% homology with peptide Leucine Arginine (pLR). The effects of the natural amidated peptide, non-amidated peptide and C-loop region of pYR on granulopoiesis and neutrophil apoptosis were investigated. All three analogues inhibited the early development of granulocyte macrophage colonies from bone marrow stem cells but did not induce apoptosis of the end stage granulocytes, the mature neutrophil. Thus, pYR is a novel member of an important and emerging new class of amphibian peptides with hemopoietic actions.

Host-defense peptides isolated from the skin secretions of the Northern red-legged frog Rana aurora aurora

Antimicrobial peptides in the skin secretions of anurans constitute a component of the innate immunity that protects the organism against invading pathogens. Four peptides with antimicrobial activity were isolated in high yield from norepinephrine-stimulated skin secretions of the Northern red-legged frog Rana aurora aurora and their primary structures determined. Ranatuerin-2AUa (GILSSFKGVAKGVAKNLAGKLLDELKCKITGC) showed potent growth-inhibitory activity against a range of Gram-positive and Gram-negative bacteria (minimum inhibitory concentrations < 20 microM) but low hemolytic activity against human erythrocytes (50% hemolysis at 290 microM). Brevinin-1AUa (FLPILAGLAAKLVPKVFCSITKKC) and brevinin-1AUb (FLPILAGLAANILPKVFCSITKKC) also showed potent antimicrobial activity but were strongly hemolytic (HC50 < 10 microM). Temporin-1AUa (FLPIIGQLLSGLL.NH2) atypically lacked a basic amino acid residue and showed very weak antimicrobial and hemolytic activity. Its biological function remains to be established. The primary structures of the antimicrobial peptides are consistent with a close phylogenetic relationship between R. aurora, Rana boylii and Rana luteiventris.

Temporins, small antimicrobial peptides with leishmanicidal activity

Leishmaniasis encompasses a wide range of infections caused by the human parasitic protozoan species belonging to the Leishmania genus. It appears frequently as an opportunistic disease, especially in virus-infected immunodepressed people. Similarly to other pathogens, parasites became resistant to most of the first-line drugs. Therefore, there is an urgent need to develop antiparasitic agents with new modes of action. Gene-encoded antimicrobial peptides are promising candidates, but so far only a few of them have shown anti-protozoa activities. Here we found that temporins A and B, 13-amino acid antimicrobial peptides secreted from the skin of the European red frog Rana temporaria, display anti-Leishmania activity at micromolar concentrations, with no cytolytic activity against human erythrocytes. To the best of our knowledge, temporins represent the shortest natural peptides having the highest leishmanicidal activity and the lowest number of positively charged amino acids (a single lysine/arginine) and maintain biological function in serum. Their lethal mechanism involves plasma membrane permeation based on the following data. (i) They induce a rapid collapse of the plasma membrane potential. (ii) They induce the influx of the vital dye SYTOX Green. (iii) They reduce intracellular ATP levels. (iv) They severely damage the membrane of the parasite, as shown by transmission electron microscopy. Besides giving us basic important information, the unique properties of temporins, as well as their membranolytic effect, which should make it difficult for the pathogen to develop resistance, suggest them as potential candidates for the future design of antiparasitic drugs with a new mode of action.

Amphibian oncology

Spontaneous neoplasia is rare in all three orders of Amphibia. Tumors are documented in most major organ systems, and some have various underlying etiologies, including viral infection, environmental contaminants, and genetic predisposition. Currently,treatment options are limited to removal of the predisposing condition(s), palliative care, surgical excision, and, when necessary,humane euthanasia may be elected. Neoplasia must be distinguished from common infectious, nonneoplastic conditions that can negatively impact population health. This article is a review of the more common types of neoplasia in amphibians, and includes clinically relevant information, such as biologic behavior,anatomy, associated etiologies, major differential diagnoses, and clinical management.

Purification and characterization of antimicrobial peptides from the skin secretions of the mink frog (Rana septentrionalis)

Skin secretions were obtained from male, female, and juvenile specimens of the mink frog (Rana septentrionalis) by electric stimulation and shown to contain 10 peptides that differentially inhibited the growth of microorganisms. The elution profiles of secretions from the three groups following reverse-phase HPLC were almost identical indicating that there were no major sexual or developmental differences in chemical composition. Four peptides of the brevinin-1 family, with potent antimicrobial activity and strong hemolytic activity, two members of ranatuerin-2 family and three members of the temporin family, were purified and characterized structurally. A 21-amino-acid C-terminally alpha-amidated peptide (GIWDTIKSMGKVFAGKILQNL.NH(2)) with broad-spectrum antimicrobial activity was also isolated from the skin secretions. This peptide shows limited structural similarity with the N-terminal region of brevinin-2 peptides previously isolated from R. temporaria skin but lacks the C-terminal cyclic heptapeptide domain associated with this family. Molecular and morphological data support the placement of R. septentrionalis in the R. catesbeiana species group, but analysis based upon the distribution of the molecular forms of the antimicrobial peptides is indicative of a closer phylogenetic relationship between R. septentrionalis and the frogs of the R. pipiens and R. boylii groups.

Antimicrobial peptides from ranid frogs: taxonomic and phylogenetic markers and a potential source of new therapeutic agents

Granular glands in the skins of frogs of the genus Rana, a widely distributed group with over 250 species, synthesize and secrete a remarkably diverse array of peptides with antimicrobial activity that are believed to have arisen as a result of multiple gene duplication events. Almost without exception, these components are hydrophobic, cationic and form an amphipathic alpha-helix in a membrane-mimetic solvent. The peptides can be grouped into families on the basis of structural similarity. To date, brevinin-1, esculentin-1, esculentin-2, and temporin peptides have been found in ranid frogs of both Eurasian and North American origin; ranalexin, ranatuerin-1, ranatuerin-2 and palustrin peptides only in N. American frogs; and brevinin-2, tigerinin, japonicin, nigrocin and melittin-related peptides only in Eurasian frogs. It is generally assumed that this structurally diversity serves to protect the organism against a wide range of pathogens but convincing evidence in support of this hypothesis is still required. The possibility that "antimicrobial peptides" fulfill additional or alternative biological functions should not be rejected. The molecular heterogeneity of the peptide families, particularly brevinin-1, brevinin-2 and ranatuerin-2, may be exploited for the purposes of unequivocal identification of specimens and for an understanding of phylogenetic interrelationships between species. The broad-spectrum antibacterial and antifungal activities of certain peptides, for example esculentin-1, ranalexin-1 and ranatuerin, together with their relatively low hemolytic activity, make them candidates for development into therapeutically useful anti-infective agents.

Antimicrobial properties of the frog skin peptide, ranatuerin-1 and its [Lys-8]-substituted analog

The predicted conformation of ranatuerin-1 (SMLSVLKNLG(10)KVGLGFVACK(20)INK QC), an antimicrobial peptide first isolated from the skin of the bullfrog Rana catesbeiana, comprises three structural domains: alpha-helix (residues 1-8), beta-sheet (residues 11-16) and beta-turn (residues 20-25). Circular dichroism studies confirm significant alpha-helical character in 50% trifluoroethanol. Replacement of Cys-19 and Cys-25 by serine resulted only in decreased antimicrobial potency but deletion of either the cyclic heptapeptide region [residues (19-25)] or the N-terminal domain [residues (1-8)] produced inactive analogs. Substitution of the glycine residues in the central domain of the [Ser-19, Ser-25] analog by lysine produced inactive peptides despite increased alpha-helical content and cationicity. The substitution Asn-8-->Lys gave a ranatuerin-1 analog with increased alpha-helicity and cationicity and increased potency against a range of Gram-positive and Gram-negative bacteria and against C. albicans but only a small increase (21%) in hemolytic activity. In contrast, increasing alpha-helicity and hydrophobicity by the substitution Asn-22-->Ala resulted in a 3.5-fold increase in hemolytic activity. Effects on antimicrobial potencies of substitutions of neutral amino acids at positions 4, 18, 22, and 24 by lysine were less marked. Strains of pathogenic E. coli from different groups showed varying degrees of sensitivity to ranatuerin-1 (MIC between 5 and 40 microM) but [Lys-8] ranatuerin-1 showed increased potency (between 2- and 8-fold; P < 0.01) against all strains. The data demonstrate that [Lys-8] ranatuerin-1 shows potential as a candidate for drug development.

Activities of temporin family peptides against the chytrid fungus (Batrachochytrium dendrobatidis) associated with global amphibian declines

Temporin A and structurally related peptides produced in amphibian dermal granular glands and in wasp venom were tested for growth inhibition of Batrachochytrium dendrobatidis, a pathogen associated with global amphibian declines. Two natural amphibian temporins, a wasp temporin, and six synthetic analogs effectively inhibited growth. Differences in potency due to amino acid substitution suggest that ability to penetrate membranes and form an alpha-helical structure is important for their effectiveness against this pathogen.

Antimicrobial peptides and protease inhibitors in the skin secretions of the crawfish frog, Rana areolata

The dorsal skin of the crawfish frog, Rana areolata, is associated with numerous prominent granular glands. Proteomic analysis of electrically stimulated skin secretions from these glands enabled the identification and characterization of eight peptides with antimicrobial and hemolytic activity belonging to the previously identified brevinin-1, temporin-1, palustrin-2, palustrin-3, esculentin-1 (two peptides), and ranatuerin-2 (two peptides) families. The primary structures of the peptides were consistent with a close phylogenetic relationship between R. areolata and the pickerel frog, Rana palustris. Three structurally related cationic, cysteine-containing peptides were identified that show sequence similarity to peptide Leucine-Arginine, a peptide with immunomodulatory and histamine-releasing properties from the skin of the northern leopard frog, Rana pipiens. The skin secretions contained a 61-amino-acid-residue peptide that inhibited porcine trypsin and possessed a 10-cysteine-residue motif that is characteristic of a protease inhibitor previously isolated from the parasitic nematode, Ascaris suum. A 48-amino-acid-residue protein containing eight cysteine residues in the whey acidic protein (WAP) motif, characteristic of elafin (skin-derived antileukoproteinase) and secretory leukocyte protease inhibitor, was also isolated. The data suggest that protease inhibitors in skin secretions may play a role complementary to cationic, amphipathic alpha-helical peptides in protecting anurans from invasions by microorganisms.

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

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

Antimicrobial peptide defenses of the Tarahumara frog, Rana tarahumarae

Populations of the Tarahumara frog Rana tarahumarae have decreased markedly in recent years in the northern part of their range. Infection by the chytrid fungus Batrachochytrium dendrobatidis has been implicated in these declines. To determine whether antimicrobial peptides in the skin provide protection against this pathogen, norepinephrine-stimulated skin secretions were tested for their ability to inhibit growth of B. dendrobatidis in vitro. After concentration, crude mixtures of skin peptides inhibited the growth of the chytrid in a concentration-dependent manner. Proteomic analysis led to the identification and characterization of three peptides belonging to the brevinin-1 family of antimicrobial peptides and three belonging to the ranatuerin-2 family. The two most abundant peptides, ranatuerin-2TRa (GIMDSIKGAAKEIAGHLLDNLKCKITGC) and brevinin-1TRa (FLPVIAGIAANVLPKLFCKLTKRC), were active against B. dendrobatidis (MIC of 50 microM for ranatuerin-2TRa and 12.5 microM for brevinin-1TRa against zoospores). These data clearly show that antimicrobial peptides in the skin secretions of the Tarahumara frog are active against B. dendrobatidis and should provide some protection against infection. Therefore, the observed susceptibility of these frogs to this pathogen in the wild may be due to the effects of additional environmental factors that impair this innate defense mechanism, leading to the observed population declines.

Antimicrobial peptides with atypical structural features from the skin of the Japanese brown frog Rana japonica

Japonicin-1 (FFPIGVFCKIFKTC) and japonicin-2 (FGLPMLSILPKALCILLKRKC), two peptides with differential growth-inhibitory activity against the Gram-negative bacterium, Escherichia coli and the Gram-positive bacterium Staphylococcus aureus, were isolated from an extract of the skin of the Japanese brown frog Rana japonica. Both peptides show little amino acid sequence similarity to previously characterized antimicrobial peptides isolated from the skins of Ranid frogs. Circular dichroism studies, however, demonstrate that japonicin-2 adopts an alpha-helical conformation in 50% trifluoroethanol in common with many other cationic antimicrobial peptides synthesized in amphibian skin. Peptides belonging to the brevinin-1, brevinin-2, and tigerinin families, previously identified in the skins of Asian Ranid frogs, were not detected but a temporin-related peptide (ILPLVGNLLNDLL.NH(2); temporin-1Ja), that atypically bears no net positive charge, was isolated from the extract. The minimum inhibitory concentrations (MICs) of the peptides against E. coli were japonicin-1, 30 microM; japonicin-2, 12 microM; and temporin-1Ja > 100 microM. The MICs against S. aureus were japonicin-1, > 100 microM; japonicin-2, 20 microM; and temporin-1Ja, > 100 microM.

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.

Multiple antimicrobial peptides and peptides related to bradykinin and neuromedin N isolated from skin secretions of the pickerel frog, Rana palustris

The skin secretions of the North American pickerel frog Rana palustris are toxic to both microorganisms and predators. A total of 22 peptides with differential growth-inhibitory activity towards bacteria and yeast were isolated from the electrostimulated secretions of R. palustris skin and were characterized structurally. Thirteen of the antimicrobial peptides belong to five of the known families previously identified in the skins of other species of Ranid frogs: brevinin-1 (3 peptides), esculentin-1 (2 peptides), esculentin-2 (1 peptide), ranatuerin-2 (6 peptides), and temporin (1 peptide). Nine peptides show little structural similarity towards other known antimicrobial peptides and so are classified in new families: palustrin-1 (4 peptides) with 27-28 amino acid residues and a cystine-bridged heptapeptide ring; palustrin-2 (3 peptides) with 31 amino acids and a cyclic heptapeptide region and palustrin-3 (2 peptides) with 48 amino acids and a cyclic hexapeptide region. Peptides belonging to the esculentin-1, esculentin-2 and palustrin-3 families are the most potent (minimal inhibitory concentrations approximately 1 microM against Escherichia coli) whereas peptides of the brevinin-1 and esculentin-2 families show the broadest spectrum of activity. As well as bradykinin that is identical to the human peptide, a further 4 peptides structurally related to [Leu(8)]bradykinin and two peptides related to neuromedin-N (the hexapeptide KKPYIL and a larger, cystine-containing form HLRRCGKKPYILMACS) were purified from the skin secretions.

Induction of synthesis of an antimicrobial peptide in the skin of the freeze-tolerant frog, Rana sylvatica, in response to environmental stimuli

An extract of skin taken from specimens of the freeze-tolerant wood frog, Rana sylvatica, that were collected from cold (<7 degrees C) ponds and maintained at 5 degrees C lacked detectable antimicrobial activity. In contrast, an extract of skin taken from specimens maintained at 30 degrees C for 3 weeks under laboratory conditions contained a high concentration (approximately 4 nmol/g) of a single antimicrobial peptide of the brevinin-1 family (FLPVVAGLAAKVLPSIICAVTKKC). The peptide inhibited growth of Escherichia coli (minimum inhibitory concentration 45 microM) and Staphylococcus aureus (minimum inhibitory concentration 7 microM). The data suggest that synthesis of the peptide is induced when the animal is in an environment that promotes the growth of microorganisms consistent with a role in the animal's defense strategy.