The chemistry and biological activities of peptides from amphibian skin secretions

From our roots, we grow

Located at the head of the Indo-Burma biodiversity hotspot, the Kunming Institute of Zoology (KIZ), Chinese Academy of Sciences (CAS), serves as China's main center for research into the diverse animal and ecological resources of southwestern China, Eastern Himalayas, and Southeast Asia. As of October 2019, it has been 60 years since the inception of KIZ. Since 1959, strong roots have been laid down by generations of researchers, allowing KIZ to grow and evolve into a comprehensive research institution renowned for its remarkable achievements in evolutionary mechanisms of animal biodiversity, animal resources protection, and sustainable utilization. It is now recognized as "a major powerhouse in evolutionary biology research in China" and is"establishing itself in the world stage" (Overseas Experts Review Committee, organized by the Bureau of Development Planning, CAS, during international evaluation in 2014).To celebrate the 60th anniversary of KIZ and the 70th anniversary of CAS, Zoological Research presents this commemorative issue, composed primarily of contributions from KIZ researchers. In addition, it is our great honor to provide here a brief retrospective of the pioneering work undertaken by the earlier scientists at KIZ and recent achievements, which will hopefully serve to motivate and inspire present and future successors.

RNA-seq analysis provides insight into molecular adaptations of Andrias davidianus

The Chinese giant salamander Andrias davidianus is regarded as an ideal model for studying local adaptations, such as longevity, tolerance to starvation, and cutaneous respiration. Transcriptome analysis is useful for studying the large and complex genomes of amphibians. Based on the coding gene set of adult A. davidianus, dozens of A. davidianus-specific genes were identified and three signaling pathway (JAK-STAT, HIF-1, and FoxO) genes were expanded as compared with other amphibians. The results of the pathway analysis of A. davidianus-specific genes indicated that the molecular adaptation of A. davidianus may have required a more rapid evolution of the immune system. Additionally, for the first time, the gene expressions in different parts of the skin tissue were compared. The results of the comparison analysis demonstrated that lateral skin could be more focused on mucus secretion, dorsal skin on immunity and melanogenesis, and abdominal skin on water and salt metabolism. This study provides the first insight into studying longevity and starvation tolerance in A. davidianus, and offers a basis for further investigation of the molecular mechanisms of adaptations in amphibians.

Unravelling the Skin Secretion Peptides of the Gliding Leaf Frog, Agalychnis spurrelli (Hylidae)

Frog skin secretions contain medically-valuable molecules, which are useful for the discovery of new biopharmaceuticals. The peptide profile of the skin secretion of Agalychnis spurrelli has not been investigated; therefore, the structural and biological characterization of its compounds signify an inestimable opportunity to acquire new biologically-active chemical scaffolds. In this work, skin secretion from this amphibian was analysed by molecular cloning and tandem mass spectrometry. Although the extent of this work was not exhaustive, eleven skin secretion peptides belonging to five peptide families were identified. Among these, we report the occurrence of two phyllokinins, and one medusin-SP which were previously reported in other related species. In addition, eight novel peptides were identified, including four dermaseptins, DRS-SP2 to DRS-SP5, one phylloseptin-SP1, and three orphan peptides. Phylloseptin-SP1 and dermaseptins-SP2 were identified in HPLC fractions based on their molecular masses determined by MALDI-TOF MS. Among the antimicrobial peptides, dermaseptin-SP2 was the most potent, inhibiting Escherichia coli, Staphylococcus aureus, and ORSA with a minimum inhibitory concentration (MIC) of 2.68 μM, and Candida albicans with an MIC of 10.71 μM, without haemolytic effects. The peptides described in this study represent but a superficial glance at the considerable structural diversity of bioactive peptides produced in the skin secretion of A. spurrelli.

Conformational analysis and in vitro immunomodulatory and insulinotropic properties of the frog skin host-defense peptide rhinophrynin-27 and selected analogs

The study investigates conformational analysis and the in vitro cytokine-mediated immunomodulatory and insulin-releasing activities of rhinophrynin-27 (ELRLPEIARPVPEVLPARLPLPALPRN; RP-27), a proline-arginine-rich peptide first isolated from skin secretions of the Mexican burrowing toad Rhinophrynus dorsalis (Rhinophrynidae). In both water and 50% trifluoroethanol-water, the peptide adopts a polyproline type II helical conformation with a high degree of deviation from the canonical collagen-like folding and a pronounced bend in the molecule at the Glu¹³ residue. Incubation of mouse peritoneal cells with RP-27 significantly (P < 0.05) inhibited production of the pro-inflammatory cytokines TNF-α and IL-1β and stimulated production of the anti-inflammatory cytokine IL-10. The peptide significantly (P < 0.01) stimulated release of insulin from BRIN-BD11 rat clonal β-cells at concentrations ≥ 1 nM while maintaining the integrity of the plasma membrane and also stimulated insulin release from isolated mouse islets at a concentration of 10^-6 M. Increasing the cationicity of RP-27 by substituting glutamic acid residues in the peptide by arginine and increasing hydrophobicity by substituting alanine residues by tryptophan did not result in analogues with increased activity with respect to cytokine production and insulin release. The combination of immunosuppressive and insulinotropic activities together with very low cytotoxicity suggests that RP-27 may represent a template for the development of an agent for use in anti-inflammatory and Type 2 diabetes therapies.

A Frog-Derived Immunomodulatory Peptide Promotes Cutaneous Wound Healing by Regulating Cellular Response

Wound healing-promoting peptides exhibit excellent therapeutic potential in regenerative medicine. However, amphibian-derived wound healing-promoting peptides and their mechanism of action remain to be further elucidated. We hereby characterized a wound healing-promoting peptide, Ot-WHP, derived from Chinese concave-eared frog Odorrana tormota. It efficiently promoted wound healing in a mouse model of full-thickness wounds. Ot-WHP significantly increased the number of neutrophils in wounds, and modestly promoted neutrophil phagocytosis and phorbol myristate acetate (PMA)-induced neutrophil extracellular trap formation. Ot-WHP also significantly increased the number of macrophages in wound sites, and directly induced chemokine, cytokine and growth factor production in macrophages by activating mitogen-activated protein kinases (MAPKs) and nuclear factor-κB (NF-κB) signaling pathways. Of note, Ot-WHP did not act as a chemoattractant for neutrophils and macrophages, suggesting its chemotactic activity depends on inducing chemoattractant production in macrophages. Besides, Ot-WHP directly promoted keratinocyte migration by enhancing integrin expression and cell adhesion. In addition, Ot-WHP significantly enhanced the cross-talk between macrophages and keratinocytes/fibroblasts by promoting keratinocyte/fibroblast proliferation, and fibroblast-to-myofibroblast transition despite having no direct effects on keratinocyte/fibroblast proliferation, and fibroblast differentiation. Collectively, Ot-WHP directly elicited the production of regulatory factors in macrophages, consequently initiated and accelerated the inflammatory phase by recruiting neutrophils and macrophages to wounds, and in turn enhanced the cross-talk between macrophages and keratinocytes/fibroblasts, additionally promoted keratinocyte migration, and finally promoted cutaneous wound healing. Our findings provide a promising immunomodulator for acute wound management and new clues for understanding the mechanism of action of amphibian-derived wound healing-promoting peptides.

Antiviral activity of Piscidin 1 against pseudorabies virus both in vitro and in vivo

Background

Swine-origin virus infection spreading widely could cause significant economic loss to porcine industry. Novel antiviral agents need to be developed to control this situation.

Methods

In this study, we evaluated the activities of five broad-spectrum antimicrobial peptides (AMPs) against several important swine-origin pathogenic viruses by TCID₅₀ assay. Plaque reduction assay and cell apoptosis assay were also used to test the activity of the peptides. Protection effect of piscidin against pseudorabies virus (PRV) was also examined in mouse model.

Results

Piscidin (piscidin 1), caerin (caerin 1.1) and maculatin (maculatin 1.1) could inhibit PRV by direct interaction with the virus particles in a dose-dependent manner and they could also protect the cells from PRV-induced apoptosis. Among the peptides tested, piscidin showed the strongest activity against PRV. Moreover, in vivo assay showed that piscidin can reduce the mortality of mice infected with PRV.

Conclusion

In vitro and in vivo experiments indicate that piscidin has antiviral activity against PRV.

Discovery and Development of Novel Anti-fungal Peptides Against Foodspoiling Fungi

BACKGROUND

Despite the extensive research carried out to develop natural antifungal preservatives for food applications, there are very limited antifungal agents available to inhibit the growth of spoilage fungi in processed foods. Scope and Approach: Therefore, this review summarizes the discovery and development of antifungal peptides using lactic acid bacteria fermentation to prevent food spoilage by fungi. The focus of this review will be on the identification of antifungal peptides, potential sources, the possible modes of action and properties of peptides considered to inhibit the growth of spoilage fungi. Key Findings and Conclusions: Antifungal peptides generated by certain lactic acid bacteria strains have a high potential for applications in a broad range of foods. The mechanism of peptides antifungal activity is related to their properties such as low molecular weight, concentration and secondary structure. The antifungal peptides were proposed to be used as bio-preservatives to reduce and/or replace chemical preservatives.

Biofilms: Novel Strategies Based on Antimicrobial Peptides

The problem of drug resistance is very worrying and ever increasing. Resistance is due not only to the reckless use of antibiotics but also to the fact that pathogens are able to adapt to different conditions and develop self-defense mechanisms such as living in biofilms; altogether these issues make the search for alternative drugs a real challenge. Antimicrobial peptides appear as promising alternatives but they have disadvantages that do not make them easily applicable in the medical field; thus many researches look for solutions to overcome the disadvantages and ensure that the advantages can be exploited. This review describes the biofilm characteristics and identifies the key features that antimicrobial peptides should have. Recalcitrant bacterial infections caused by the most obstinate bacterial species should be treated with a strategy to combine conventional peptides functionalized with nano-tools. This approach could effectively disrupt high density infections caused by biofilms. Moreover, the importance of using in vivo non mammalian models for biofilm studies is described. In particular, here we analyze the use of amphibians as a model to substitute the rodent model.

Bioevaluation of Ranatuerin-2Pb from the Frog Skin Secretion of Rana pipiens and its Truncated Analogues

Antimicrobial peptides (AMPs) are considered as a promising agent to overcome the drug-resistance of bacteria. Large numbers of AMPs have been identified from the skin secretion of Rana pipiens, including brevinins, ranatuerins, temporins and esculentins. In this study, the cDNA precursor of a broad-spectrum antimicrobial peptide, ranatuerin-2Pb, was cloned and identified. Additionally, two truncated analogues, RPa and RPb, were synthesised to investigate the structure-activity relationship of ranatuerin-2Pb. RPa lost antimicrobial activity against Candida albicans, MRSA, Enterococcus faecalis and Pseudomonas aeruginosa, while RPb retained its broad-spectrum antimicrobial activity. Additionally, ranatuerin-2Pb, RPa and RPb demonstrated inhibition and eradication effects against Staphylococcus aureus biofilm. RPb showed a rapid bacterial killing manner via membrane permeabilization without damaging the cell membrane of erythrocytes. Moreover, RPb decreased the mortality of S. aureus infected Galleria mellonella larvae. Collectively, our results suggested that RPb may pave a novel way for natural antimicrobial drug design.

Rational Design of Short Peptide Variants by Using Kunitzin-RE, an Amphibian-Derived Bioactivity Peptide, for Acquired Potent Broad-Spectrum Antimicrobial and Improved Therapeutic Potential of Commensalism Coinfection of Pathogens

Commensalism coinfection of pathogens has seriously jeopardized human health. Currently, Kunitzin-RE, as an amphibian-derived bioactivity peptide, is regarded as a potential antimicrobial candidate. However, its antimicrobial properties were unsatisfactory. In this study, a set of shortened variants of Kunitzin-RE was developed by the interception of a peptide fragment and single-site mutation to investigate the effect of chain length, positive charge, hydrophobicity, amphipathicity, and secondary structure on antimicrobial properties. Among them, W8 (AARIILRWRFR) significantly broadened the antimicrobial spectrum and showed the highest antimicrobial activity (GM_all = 2.48 μM) against all the fungi and bacteria tested. Additionally, W8 showed high cell selectivity and salt tolerance in vitro, whereas it showed high effectiveness against mice keratitis cause by infection by C. albicans 2.2086. Additionally, it also had obviously lipopolysaccharide-binding ability and a potent membrane-disruptive mechanism. Overall, these findings contributed to the design of short antimicrobial peptides and to combat the serious threat of commensalism coinfection of pathogens.

Production of a Recombinant Dermaseptin Peptide in Nicotiana tabacum Hairy Roots with Enhanced Antimicrobial Activity

Expression of strong antimicrobial peptides in plants is of great interest to combat a wide range of plant pathogens. To bring the Dermaseptin B1 (DrsB1) peptide to the intimate contact of the plant pathogens cell wall surface, the DrsB1 encoding sequence was fused to the C-terminal part of the two copies of the chitin-binding domain (CBD) of the Avr4 effector protein and used for Agrobacterium rhizogenes-mediated transformation. The expression of the recombinant protein in the tobacco hairy roots (HRs) was confirmed by molecular analysis. Antimicrobial activity analysis of the recombinant protein purified from the transgenic HRs showed that the (CBD)2-DrsB1 recombinant protein had a significant (p < 0.01) antimicrobial effect on the growth of different fungal and bacterial pathogens. The results of this study indicated that the recombinant protein had a higher antifungal activity against chitin-producing Alternaria alternata than Pythium spp. Scanning electron microscopy images demonstrated that the recombinant protein led to fungal hypha deformation, fragmentation, and agglutination of growing hypha, possibly by dissociating fungal cell wall components. In vitro evidences suggest that the expression of the (CBD)2-DrsB1 recombinant protein in plants by generating transgenic lines is a promising approach to produce disease-resistant plants, resistance to chitin-producing pathogenic fungi.

Amphibian skin extracts as source of bioactive multi-target agents against different pathways of Alzheimer's disease

Natural products represent a rich source of bioactive compounds that have been historically used to obtain substances with great medicinal potential. The skin of anuran amphibians is a rich source of compounds with a wide range of biological activity. Alzheimer's disease (AD) is a complex disease associated with all kind of different pathways, making their simultaneous modulation necessary. Nowadays anti-AD treatments are focused on enzymatic inhibitors. Here in we report the activity of skin extracts from nine species from Argentina, belonging to three anuran families, as inhibitors of AChE, BChE and MAO-B enzymes. The extracts also showed antioxidant activities, acting as multi-target on four important pathways of AD.

Identification and Characterization of a Novel Gene-encoded Antioxidant Peptide from Odorous Frog Skin

Background:

Amphibian skin plays an essential role in protecting organisms from harmful external factors such as UV radiation. How amphibians protect themselves from reactive oxygen species following long-term sun exposure is an important and interesting question. Amphibian skins possess a novel antioxidant system composed of various Antioxidant Peptides (AOPs), which maintain redox homeostasis. However, only a few AOPs have been identified so far.

Methods:

Using combinational methods of peptidomics and genomics, we characterized a novel gene-encoded antioxidant peptide (herein named OA-VI12) from Odorrana andersonii skin secretions, which was produced by the post-translational processing of a 59-residue prepropeptide. The amino acid sequence of the OA-V112 was 'VIPFLACRPLGL', with a molecular mass of 1298.6 Da and no observed post-transcriptional modifications. Functional analysis demonstrated that OA-VI12 was capable of scavenging ABTS+, DPPH, NO and decreasing the Fe3+ production.

Results:

We determined that the C7 amino acid was responsible for ABTS+ and Fe3+ scavenging, activities, the F4, C7, and P9 amino acids were crucial for DPPH scavenging activity, and the P9 amino acid was responsible for NO scavenging activity. Unlike several other amphibian peptides, OA-VI12 did not accelerate wound healing in a full-thickness skin-wound mouse model and did not demonstrate direct microbial killing. Here, we identified and named a novel gene-encoded antioxidant peptide from the skin secretions of an odorous frog species, which may assist in the development of potential antioxidant candidates.

Conclusion:

This study may help improve our understanding of the molecular basis of amphibians’ adaptation to environments experiencing long-term UV radiation.

Immunomodulatory, insulinotropic, and cytotoxic activities of phylloseptins and plasticin-TR from the Trinidanian leaf frog Phyllomedusa trinitatis

The aim of the study was to determine the in vitro immunomodulatory, cytotoxic, and insulin-releasing activities of seven phylloseptin-TR peptides and plasticin-TR, first isolated from the frog Phyllomedusa trinitatis. The most cationic peptides, phylloseptin-1.1TR and phylloseptin-3.1TR, showed greatest cytotoxic potency against A549, MDA-MB231, and HT-29 human tumor-derived cells and against mouse erythrocytes. Phylloseptin-4TR was the most hydrophobic and the most effective peptide at inhibiting production of the proinflammatory cytokines TNF-α and IL-1β by mouse peritoneal cells but was without effect on production of the antiinflammatory cytokine IL-10. Phylloseptin-2.1TR and phylloseptin-3.3TR were the most effective at stimulating the production of IL-10. The noncytotoxic peptide, plasticin-TR, inhibited production of TNF-α and IL-1β but was without effect on IL-10 production. The results of CD spectroscopy suggest that the different properties of plasticin-TR compared with the immunostimulatory activities of the previously characterized plasticin-L1 from Leptodactylus laticeps may arise from greater ability of plasticin-TR to oligomerize and adopt a stable helical conformation in a membrane-mimetic environment. All peptides stimulated release of insulin from BRIN-BD11 rat clonal β cells with phylloseptin-3.2TR being the most potent and effective and phylloseptin-2.1TR the least effective suggesting that insulinotropic potency correlates inversely with helicity. The study has provided insight into structure-activity relationships among the phylloseptins. The combination of immunomodulatory and insulinotropic activities together with low cytotoxicity suggests that phylloseptin-3.3TR and plasticin-TR may represent templates for the development of agents for use in antiinflammatory and type 2 diabetes therapies.

Activation of the Bile Acid Pathway and No Observed Antimicrobial Peptide Sequences in the Skin of a Poison Frog

The skin secretions of many frogs have genetically-encoded, endogenous antimicrobial peptides (AMPs). Other species, especially aposematic poison frogs, secrete exogenously derived alkaloids that serve as potent defense molecules. The origins of these defense systems are not clear, but a novel bile-acid derived metabolite, tauromantellic acid, was recently discovered and shown to be endogenous in poison frogs (Mantella, Dendrobates, and Epipedobates). These observations raise questions about the evolutionary history of AMP genetic elements, the mechanism and function of tauromatellic acid production, and links between these systems. To understand the diversity and expression of AMPs among frogs, we assembled skin transcriptomes of 13 species across the anuran phylogeny. Our analyses revealed a diversity of AMPs and AMP expression levels across the phylogenetic history of frogs, but no observations of AMPs in Mantella We examined genes expressed in the bile-acid metabolic pathway and found that CYP7A1 (Cytochrome P450), BAAT (bile acid-CoA: amino acid N-acyltransferase), and AMACR (alpha-methylacyl-CoA racemase) were highly expressed in the skin of M. betsileo and either lowly expressed or absent in other frog species. In particular, CYP7A1 catalyzes the first reaction in the cholesterol catabolic pathway and is the rate-limiting step in regulation of bile acid synthesis, suggesting unique activation of the bile acid pathway in Mantella skin. The activation of the bile acid pathway in the skin of Mantella and the lack of observed AMPs fuel new questions about the evolution of defense compounds and the ectopic expression of the bile-acid pathway.

Peptides for Skin Protection and Healing in Amphibians

Amphibian skin is not to be considered a mere tegument; it has a multitude of functions related to respiration, osmoregulation, and thermoregulation, thus allowing the individuals to survive and thrive in the terrestrial environment. Moreover, amphibian skin secretions are enriched with several peptides, which defend the skin from environmental and pathogenic insults and exert many other biological effects. In this work, the beneficial effects of amphibian skin peptides are reviewed, in particular their role in speeding up wound healing and in protection from oxidative stress and UV irradiation. A better understanding of why some species seem to resist several environmental insults can help to limit the ongoing amphibian decline through the development of appropriate strategies, particularly against pathologies such as viral and fungal infections.

Peptidomic analysis of the host-defense peptides in skin secretions of Rana graeca provides insight into phylogenetic relationships among Eurasian Rana species

Peptidomic analysis of norepinephrine-stimulated skin secretions from the Greek stream frog Rana graeca Boulenger, 1891 led to the identification and structural characterization of a range of host-defense peptides. These comprised brevinin-1GRa, brevinin-1GRb and an N-terminally extended form of brevinin-1GRb, ranatuerin-2GR together with its oxidized form and (11-28) fragment, temporin-GRa, temporin-GRb and its non-amidated form, and a melittin-related peptide, MRP-GR and its (1-18) fragment. The most abundant peptide, MRP-GR significantly (P < 0.001) stimulated insulin release from BRIN-BD11 clonal β-cells at concentrations ≥0.1 nM. Rana graeca (formerly Rana graeca graeca) and the morphologically similar Italian stream frog Rana italica Dubois, 1987 (formerly Rana graeca italica) were originally regarded as sub-species. However, the primary structures of the host defense peptides from both frogs support the claim based upon comparisons of the nucleotide sequences of S1 satellite DNA that R. graeca and R. italica are separate species. Cladistic analyses based upon the primary structures of the brevinin-1 and ranatuerin-2 peptides from Eurasian frogs indicate a close phylogenetic relationship between R. graeca and Rana latastei whereas R. italica is most closely related to Rana dalmatina.

Skin innate immunity of diskless-fingered odorous frogs (Odorrana grahami) with spatial-temporal variations

The skin innate immunities of diskless-fingered odorous frogs (Odorrana grahami) from three populations were investigated. The antimicrobial capacities of skin secretions against the 60 representative environmental bacterial strains were evaluated using the values of the minimum inhibitory concentration (MIC) equivalents, which were defined as the volumes of antimicrobial solution just inhibiting the tested bacteria per 1 cm² of surface area, from 0.06 to 9.10 mL/cm². Our results revealed significantly different skin antimicrobial capacities among the three populations: Mianning < Huili < Kunming. Within the frog population, the skin antimicrobial capacities are highly variable depending on the season: in Mianning frogs, summer < autumn and spring; in Huili frogs, spring < autumn < summer; in Kunming frogs, autumn < spring < summer. The animal density and body mass significantly impacted the skin antimicrobial capacity, while the sex ratio and soil or water bacterial counts did not.

Functional Characterization of a Novel Lipopolysaccharide-Binding Antimicrobial and Anti-Inflammatory Peptide in Vitro and in Vivo

Antimicrobial peptides (AMPs) are key components of host immune defense of vertebrates against microbial invasions. Here, we report a new AMP (esculentin-1GN) characterized from the skin of the frog Hylarana guentheri. Esculentin-1GN (GLFSKKGGKGGKSWIKGVFKGIKGIGKEVGGDVIRTGIEIAACKIKGEC) with high amphipathic α-helical structure in membrane-mimetic environments has the microbial-killing activity by destruction of the cell membrane. Moreover, esculentin-1GN inhibits LPS-induced expression of proinflammatory nitric oxide, interleukin-1β, interleukin-6, and tumor necrosis factor while it enhances expression of interleukin-10. Furthermore, esculentin-1GN can bind to d-(+)-galacturonic acid and LPS. Meanwhile, esculentin-1GN suppresses the activation of inflammatory response pathway induced by LPS. In addition, esculentin-1GN significantly reduces acute inflammation in carrageenan-induced mice paw. Taken together, the novel LPS-binding esculentin-1GN with antimicrobial and anti-inflammatory activities will be an excellent temple for designing new antibiotic formulations.

Identification of Arenin, a Novel Kunitz-Like Polypeptide from the Skin Secretions of Dryophytes arenicolor

Amphibian skin secretions are enriched with complex cocktails of bioactive molecules such as proteins, peptides, biogenic amines, alkaloids guanidine derivatives, steroids and other minor components spanning a wide spectrum of pharmacological actions exploited for centuries in folk medicine. This study presents evidence on the protein profile of the skin secretions of the canyon tree frog, Dryophytes arenicolor. At the same time, it presents the reverse-phase liquid chromatography isolation, mass spectrometry characterization and identification at mRNA level of a novel 58 amino acids Kunitz-like polypeptide from the skin secretions of Dryophytes arenicolor, arenin. Cell viability assays performed on HDFa, CaCo2 and MCF7 cells cultured with different concentrations of arenin showed a discrete effect at low concentrations (2, 4, 8 and 16 µg/mL) suggesting a multi-target interaction in a hormetic-like dose-response. Further work is required to investigate the mechanisms underlying the variable effect on cell viability produced by different concentrations of arenin.

A novel antimicrobial peptide, Ranatuerin-2PLx, showing therapeutic potential in inhibiting proliferation of cancer cells

Antimicrobial peptides are a promising resource for developing novel antibiotic and even anticancer drugs. Here, a 28-mer polypeptide, Ranatuerin-2PLx (R2PLx), was identified from lyophilised skin secretions. The chemically synthetic replicates exhibited moderate and broadspectrum antimicrobial effect against various microorganisms including methicillin-resistant Staphylococcus aureus (MRSA, minimal inhibitory concentration = 256 µM). In addition, R2PLx was found to inhibit the proliferation of several tumour cells, especially showing more potent effect on prostate cancer cell, PC-3. The early cell apoptosis was observed in 6 h by Annexin V-FITC/propidium iodide staining, as well as the activation of Caspase-3 at 5 µM peptide concentration. R2PLx may therefore be promising for developing new therapeutic approach for cancer treatment. Moreover, the artificial deficiency of conserved rana-box loop or net positive charge in C-terminal domain notably reduced the biological activities of the truncated and substituted isoforms, respectively, suggesting for maintaining their biological potency of ranatuerin family requires both cysteine-bridged segment and cationincity within the loop domain in C-terminus.

The defensive system of tree frog skin identified by peptidomics and RNA sequencing analysis

The diversity of defensive peptides from skin of amphibians has been demonstrated. These peptides may have resulted from the diversity of microorganisms encountered by amphibians. In this study, peptidomics and RNA sequencing analyses were used to study deeply the defensive peptides of the skin secretions from Polypedates megacephalus. A total of 99 defensive peptides have been identified from the skin secretions. Among these peptides, 3 peptides were myotropical peptides and 34 peptides classified as protease inhibitor peptides. 5 lectins, 8 antimicrobial peptides, 26 immunomodulatory peptides, 10 wound-healing peptides and 13 other bioactive peptides were identified as belonging to the innate immune system. One antimicrobial peptide Pm-amp1 showed high similarity to antimicrobial peptide marcin-18. This peptide was successfully expressed and showed moderate activity against four tested strains. These identified peptides highlight the extensive diversity of defensive peptides and provide powerful tools to understand the defense weapon of frog.

Insulinotropic activity of the host-defense peptide frenatin 2D: Conformational, structure-function and mechanistic studies

Of four naturally occurring frenatin peptides tested, frenatin 2D (DLLGTLGNLPLPFI.NH₂) from Discoglossus sardus was the most potent and effective in producing concentration-dependent stimulation of insulin release from BRIN-BD11 rat clonal β-cells without displaying cytotoxicity. The peptide also stimulated insulin release from 1.1B4 human-derived clonal β-cells and isolated mouse islets and improved glucose tolerance concomitant with increased circulating insulin concentrations in mice following intraperitoneal administration. The insulinotropic activity of frenatin 2D was not associated with membrane depolarization or an increase in intracellular [Ca²⁺] but incubation of the peptide (1 μM) with BRIN-BD11 cells produced a modest, but significant (P < 0.05), increase in cAMP production. Stimulation of insulin release was abolished in protein kinase A-downregulated cells but maintained in protein kinase C-downregulated cells. Circular dichroism studies showed that, in the presence of dodecylphosphocholine micelles, frenatin 2D exhibited a helical content of 35% and a turn content of 28%. Substitution of the Thr⁵, Asn⁸, Pro¹⁰, and Ile¹⁴ residues in frenatin-2D by Trp and interchange of Pro¹² and Phe¹³ led to loss of insulinotropic activity but the [D1W] and [G7W] analogues were as potent and effective as the native peptide. Frenatin 2D (1 μM) also stimulated proliferation of BRIN-BD11 cells and provided significant protection of the cells against cytokine-induced apoptosis. It is concluded that the insulinotropic activity of frenatin 2D is mediated predominantly, if not exclusively, by the K_ATP channel-independent pathway.

Extensive characterization and differential analysis of endogenous peptides from Bombyx batryticatus using mass spectrometric approach

Bombyx batryticatus, the dried larva of Bombyx mori L. (4th-5th instars) infected with Beauveria bassiana Vuill, is an important animal-derived medicine effective against several diseases. The metamorphosis of silkworm can result insignificant changes in the levels of proteins and polypeptides in the 4^th and 5^th instar larvae. Here, we performed extensive characterization of Bombyx batryticatus peptides, including polypeptides containing cysteines, using an MS-based data mining strategy. A total of 779 peptides with various PTMs (post-translational modifications) were identified through database search and de novo sequencing. Some of these peptides might have important biological activities. Besides, the differential analysis of polypeptides between the head and body of Bombyx batryticatus was performed to provide a clinical basis for rational use of the drugs derived from it. This study illustrates the abundance and sequences of endogenous Bombyx batryticatus polypeptides, and thus, provides potential candidates for the screening of active compounds for future biological research and drug discovery studies.

Identification and characterization of two novel cathelicidins from the frog Odorrana livida

Antimicrobial peptides (AMPs) are a group of gene-encoded small peptides that play pivotal roles in the host immune system of multicellular organisms. Cathelicidins are an important family of AMPs that exclusively exist in vertebrates. Many cathelicidins have been identified from mammals, birds, reptiles and fish. To date, however, cathelicidins from amphibians are poorly understood. In the present study, two novel cathelicidins (OL-CATH1 and 2) were identified and studied from the odorous frog Odorrana livida. Firstly, the cDNAs encoding the OL-CATHs (780 and 735 bp in length, respectively) were successfully cloned from a lung cDNA library constructed for the frog. Multi-sequence alignment was carried out to analyze differences between the precursors of the OL-CATHs and other representative cathelicidins. Mature peptide sequences of OL-CATH1 and 2 were predicted (33 amino acid residues) and their secondary structures were determined (OL-CATH1 showed a random-coil conformation and OL-CATH2 demonstrated a-helical conformation). Furthermore, OL-CATH1 and 2 were chemically synthesized and their in vitro functions were determined. Antimicrobial and bacterial killing kinetic analyses indicated that OL-CATH2 demonstrated relatively moderate and rapid antimicrobial potency and exhibited strong anti-inflammatory activity. At very low concentrations (10 μg/mL), OL-CATH2 significantly inhibited the lipopolysaccharide (LPS)-induced transcription and production of pro-inflammatory cytokines TNF-a, IL-1b and IL-6 in mouse peritoneal macrophages. In contrast, OL-CATH1 did not exhibit any detectable antimicrobial or anti-inflammatory activities. Overall, identification of these OL-CATHs from O. livida enriches our understanding of the functions of cathelicidins in the amphibian immune system. The potent antimicrobial and anti-inflammatory activities of OL-CATH2 highlight its potential as a novel candidate in anti-infective drug development.

Skin glands of an aquatic salamander vary in size and distribution and release antimicrobial secretions effective against chytrid fungal pathogens

Amphibian skin is unique among vertebrate classes, containing a large number of multicellular exocrine glands that vary among species and have diverse functions. The secretions of skin glands contain a rich array of bioactive compounds including antimicrobial peptides (AMPs). Such compounds are important for amphibian innate immune responses and may protect some species from chytridiomycosis, a lethal skin disease caused by the fungal pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). While the bioactivity of skin secretions against Bd has been assessed for many amphibian taxa, similar studies are lacking for Bsal, a chytrid fungus that is especially pathogenic for salamanders. We studied the skin glands and their potential functions in an aquatic salamander, the three-toed amphiuma (Amphiuma tridactylum). Skin secretions of captive adult salamanders were analyzed by RP-HPLC and tested against the growth of Bd and Bsal using in vitro assays. We found that compounds within collected skin secretions were similar between male and female salamanders and inhibited the growth of Bd and Bsal. Thus, skin secretions that protect against Bd may also provide protection against Bsal. Histological examination of the skin glands of preserved salamanders revealed the presence of enlarged granular glands concentrated within caudal body regions. A site of potential gland specialization was identified at the tail base and may indicate specialized granular glands related to courtship and communication.

Selection and environmental adaptation along a path to speciation in the Tibetan frog Nanorana parkeri

Tibetan frogs, Nanorana parkeri, are differentiated genetically but not morphologically along geographical and elevational gradients in a challenging environment, presenting a unique opportunity to investigate processes leading to speciation. Analyses of whole genomes of 63 frogs reveal population structuring and historical demography, characterized by highly restricted gene flow in a narrow geographic zone lying between matrilines West (W) and East (E). A population found only along a single tributary of the Yalu Zangbu River has the mitogenome only of E, whereas nuclear genes of W comprise 89-95% of the nuclear genome. Selection accounts for 579 broadly scattered, highly divergent regions (HDRs) of the genome, which involve 365 genes. These genes fall into 51 gene ontology (GO) functional classes, 14 of which are likely to be important in driving reproductive isolation. GO enrichment analyses of E reveal many overrepresented functional categories associated with adaptation to high elevations, including blood circulation, response to hypoxia, and UV radiation. Four genes, including DNAJC8 in the brain, TNNC1 and ADORA1 in the heart, and LAMB3 in the lung, differ in levels of expression between low- and high-elevation populations. High-altitude adaptation plays an important role in maintaining and driving continuing divergence and reproductive isolation. Use of total genomes enabled recognition of selection and adaptation in and between populations, as well as documentation of evolution along a stepped cline toward speciation.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

The anuran skin peptide bradykinin mediates its own absorption across epithelial barriers of the digestive tract

When faced with a potential predator, a wide range of frog species secrete a mixture of peptide toxins from their skin to defend themselves. We have recently shown that antimicrobial peptides (AMPs) in a frog's defensive poison enhance the uptake of these peptides across epithelia, thereby speeding up the process of predator intoxication. This study provides evidence that bradykinin, a widespread peptide toxin in anurans (frogs), is capable to pass through epithelial barriers independent of this delivery system. We quantified bradykinin peptides secreted by Bombina orientalis during acute stress, and found that at biologically relevant concentrations, bradykinin passage across model epithelia occurs even in the absence of AMPs. Monitoring of transepithelial electric resistance showed that bradykinin treatment caused a subtle yet prolonged reduction in barrier function, indicating that the peptide itself is capable to increase the permeability of epithelia. Yet, bradykinin does not cause cells to leak lactate dehydrogenase, suggesting that it does not damage cell membranes. Moreover, imaging of bradykinin-treated monolayers shows no endocytosis of fluorescent propidium iodide, indicating that the peptide does not perforate cell membranes at smaller scale and therefore is unlikely to cross epithelia via a transcellular passage. Together, these observations suggest that bradykinin, unlike other amphibian neuropeptide toxins, mediates its own passage across mucosal barriers, possibly through a paracellular route. This "self-administering" property, combined with the fact that bradykinins can potently disturb multiple physiological processes, could explain why these peptides are one of the most widespread antipredator peptides in the defensive secretions of frogs.

EThcD Discrimination of Isomeric Leucine/Isoleucine Residues in Sequencing of the Intact Skin Frog Peptides with Intramolecular Disulfide Bond

Our scientific interests involve de novo sequencing of non-tryptic natural amphibian skin peptides including those with intramolecular S-S bond by means of exclusively mass spectrometry. Reliable discrimination of the isomeric leucine/isoleucine residues during peptide sequencing by means of mass spectrometry represents a bottleneck in the workflow for complete automation of the primary structure elucidation of these compounds. MS³ is capable of solving the problem. Earlier we demonstrated the advanced efficiency of ETD-HCD method to discriminate Leu/Ile in individual peptides by consecutive application of ETD to the polyprotonated peptides followed by HCD applied to the manually selected primary z-ions with the targeted isomeric residues at their N-termini and registration of the characteristic w-ions. Later this approach was extended to deal with several (4-7) broad band mass ranges, without special isolation of the primary z-ions. The present paper demonstrates an advanced version of this method when EThcD is applied in the whole mass range to a complex mixture of natural non-tryptic peptides without their separation and intermediate isolation of the targeted z-ions. The proposed EThcD method showed over 81% efficiency for the large natural peptides with intact disulfide ring, while the interfering process of radical site migration is suppressed. Due to higher speed and sensitivity, the proposed EThcD approach facilitates the analytical procedure and allows for the automation of the entire experiment and data processing. Moreover, in some cases it gives a chance to establish the nature of the residues in the intact intramolecular disulfide loops. Graphical Abstract ᅟ.

NMR structures in different membrane environments of three ocellatin peptides isolated from Leptodactylus labyrinthicus

The peptides ocellatin-LB1, -LB2 and -F1 have previously been isolated from anurans of the Leptodactylus genus and the sequences are identical from residue 1-22, which correspond to ocellatin-LB1 sequence (GVVDILKGAAKDIAGHLASKVM-NH₂), whereas ocellatin-LB2 carries an extra N and ocellatin-F1 extra NKL residues at their C-termini. These peptides showed different spectra of activities and biophysical investigations indicated a direct correlation between membrane-disruptive properties and antimicrobial activities, i.e. ocellatin-F1 > ocellatin-LB1 > ocellatin-LB2. To better characterize their membrane interactions, we report here the detailed three-dimensional NMR structures of these peptides in TFE-d₂:H₂O (60:40) and in the presence of zwitterionic DPC-d₃₈ and anionic SDS-d₂₅ micellar solutions. Although the three peptides showed significant helical contents in the three mimetic environments, structural differences were noticed. When the structures of the three peptides in the presence of DPC-d₃₈ micelles are compared to each other, a more pronounced curvature is observed for ocellatin-F1 and the bent helix, with the concave face composed mostly of hydrophobic residues, is consistent with the micellar curvature and the amphipathic nature of the molecule. Interestingly, an almost linear helical segment was observed for ocellatin-F1 in the presence of SDS-d₂₅ micelles and the conformational differences in the two micellar environments are possibly related to the presence of the extra Lys residue near the peptide C-terminus, which increases the affinity of ocellatin-F1 to anionic membranes in comparison with ocellatin-LB1 and -LB2, as proved by isothermal titration calorimetry. To our knowledge, this work reports for the first time the three-dimensional structures of ocellatin peptides.

Discovery of Distinctin-Like-Peptide-PH (DLP-PH) From the Skin Secretion of Phyllomedusa hypochondrialis, a Prototype of a Novel Family of Antimicrobial Peptide

Amphibian skin secretions are an important treasure house of bioactive antimicrobial peptides (AMPs). Despite having been the focus of decades of research in this context, investigations of phyllomedusine frogs continue to identify new AMPs from their skin secretions. In this study, the prototype of a novel family of AMP distinctin-like-peptide-PH (DLP-PH) was identified from the skin secretion of the otherwise well-studied Tiger-Legged Tree Frog Phyllomedusa hypochondrialis through cloning of its precursor-encoding cDNA from a skin secretion-derived cDNA library by a 3′-rapid amplification of cDNA ends (RACE) strategy. Subsequently, the mature peptide was isolated and characterized using reverse-phase HPLC and MS/MS fragmentation sequencing. DLP-PH adopted an α-helical conformation in membrane mimetic solution and demonstrated unique structural features with two distinct domains that differed markedly in their physiochemical properties. Chemically synthesized replicates of DLP-PH showed antimicrobial activity against planktonic bacterial and yeast cells, but more potent against Escherichia coli at 32 μg/mL. However, DLP-PH showed much weaker inhibitory activity against the growth of sessile cells in biofilms. In addition, DLP-PH exhibited anti-proliferative activity against human cancer cell lines, H157, and PC3, but with no major toxicity against normal human cell, HMEC-1. These combined properties make DLP-PH deserving further study as an antimicrobial agent and further investigations of its structure-activity relationship could provide valuable new insights into drug lead candidates for antimicrobial and/or anti-cancer purposes.

Structure and function of a novel antioxidant peptide from the skin of tropical frogs

The amphibian skin plays an important role protecting the organism from external harmful factors such as microorganisms or UV radiation. Based on biorational strategies, many studies have investigated the cutaneous secretion of anurans as a source of bioactive molecules. By a peptidomic approach, a novel antioxidant peptide (AOP) with in vitro free radical scavenging ability was isolated from Physalaemus nattereri. The AOP, named antioxidin-I, has a molecular weight [M+H]⁺ = 1543.69Da and a TWYFITPYIPDK primary amino acid sequence. The gene encoding the antioxidin-I precursor was expressed in the skin tissue of three other Tropical frog species: Phyllomedusa tarsius, P. distincta and Pithecopus rohdei. cDNA sequencing revealed highly homologous regions (signal peptide and acidic region). Mature antioxidin-I has a novel primary sequence with low similarity compared with previously described amphibian's AOPs. Antioxidin-I adopts a random structure even at high concentrations of hydrophobic solvent, it has poor antimicrobial activity and poor performance in free radical scavenging assays in vitro, with the exception of the ORAC assay. However, antioxidin-I presented a low cytotoxicity and suppressed menadione-induced redox imbalance when tested with fibroblast in culture. In addition, it had the capacity to substantially attenuate the hypoxia-induced production of reactive oxygen species when tested in hypoxia exposed living microglial cells, suggesting a potential neuroprotective role for this peptide.

Peptides from frog skin with potential for development into agents for Type 2 diabetes therapy

Several frog skin peptides, first identified as result of their antimicrobial or immunomodulatory activities, have subsequently been shown to stimulate insulin release both in vitro and in vivo and so show potential for development into incretin-based drugs for treatment of patients with Type 2 diabetes mellitus. However, their therapeutic potential as anti-diabetic agents is not confined to this activity as certain frog skin-derived peptides, such as magainin-AM2 and CPF-SE1 and analogs of hymenochirin-1B, tigerinin-1R, and esculentin-2CHa, have been shown to increase insulin sensitivity, promote β-cell proliferation, suppress pancreatic and circulating glucagon concentrations, improve the lipid profile, and selectively alter expression of genes involved in insulin secretion and action in mice with diet-induced obesity, insulin resistance and impaired glucose tolerance. This review assesses the therapeutic possibilities of peptides from frogs belonging to the Pipidae, Dicroglossidae, and Ranidae families, focusing upon work that has been carried out since 2014.

Cathelicidin-OA1, a novel antioxidant peptide identified from an amphibian, accelerates skin wound healing

Cathelicidins play pivotal roles in host defense. The discovery of novel cathelicidins is important research; however, despite the identification of many cathelicidins in vertebrates, few have been reported in amphibians. Here we identified a novel cathelicidin (named cathelicidin-OA1) from the skin of an amphibian species, Odorrana andersonii. Produced by posttranslational processing of a 198-residue prepropeptide, cathelicidin-OA1 presented an amino acid sequence of 'IGRDPTWSHLAASCLKCIFDDLPKTHN' and a molecular mass of 3038.5 Da. Functional analysis showed that, unlike other cathelicidins, cathelicidin-OA1 demonstrated no direct microbe-killing, acute toxicity and hemolytic activity, but did exhibit antioxidant activity. Importantly, cathelicidin-OA1 accelerated wound healing against human keratinocytes (HaCaT) and skin fibroblasts (HSF) in both time- and dose-dependent manners. Notably, cathelicidin-OA1 also showed wound-healing promotion in a mouse model with full-thickness skin wounds, accelerating re-epithelialization and granulation tissue formation by enhancing the recruitment of macrophages to the wound site, inducing HaCaT cell proliferation and HSF cell migration. This is the first cathelicidin identified from an amphibian that shows potent wound-healing activity. These results will help in the development of new types of wound-healing agents and in our understanding of the biological functions of cathelicidins.

The role of d-allo-isoleucine in the deposition of the anti-Leishmania peptide bombinin H4 as revealed by 31P solid-state NMR, VCD spectroscopy, and MD simulation

Bombinin H4 is an antimicrobial peptide that was isolated from the toad Bombina variegata. Bombinin H family peptides are active against gram-positive, gram-negative bacteria, and fungi as well as the parasite Leishmania. Among them, bombinin H4 (H4), which contains d-allo-isoleucine (d-allo-Ile) as the second residue in its sequence, is the most active, and its l-isomer is bombinin H2 (H2). H4 has a significantly lower LC50 than H2 against Leishmania. However, the atomic-level mechanism of the membrane interaction and higher activity of H4 has not been clarified. In this work, we investigated the behavior of the conformations and interactions of H2 and H4 with the Leishmania membrane using ³¹P solid-state nuclear magnetic resonance (NMR), vibrational circular dichroism (VCD) spectroscopy, and molecular dynamics (MD) simulations. The generation of isotropic ³¹P NMR signals depending on the peptide concentration indicated the abilities of H2 and H4 to exert antimicrobial activity via membrane disruption. The VCD experiment and density functional theory calculation confirmed the different stability and conformations of the N-termini of H2 and H4. MD simulations revealed that the N-terminus of H4 is more stable than that of H2 in the membrane, in line with the VCD experiment data. VCD and MD analyses demonstrated that the first l-Ile and second d-allo-Ile of H4 tend to take a cis conformation. These residues function as an anchor and facilitate the easy winding of the helical conformation of H4 in the membrane. It may assist to quickly reach to the threshold concentration of H4 on the Leishmania membrane. This article is part of a Special Issue entitled: d-Amino acids: biology in the mirror, edited by Dr. Loredano Pollegioni, Dr. Jean-Pierre Mothet and Dr. Molla Gianluca.

Identification of Components in Frog Skin Secretions with Therapeutic Potential as Antidiabetic Agents

Several peptides that were first identified on the basis of their antimicrobial or immunomodulatory properties have subsequently shown potential for development into agents for the treatment of patients with Type 2 diabetes. A strategy is presented for the isolation and characterization of such peptides in norepinephrine-stimulated skin secretions from a range of frog species. The methodology involves fractionation of the secretions by reversed-phase HPLC, identification of fractions containing components that stimulate the rate of release of insulin from BRIN-BD11 clonal β-cells without simultaneously stimulating the release of lactate dehydrogenase, identification of active peptides in the mass range 1-6 kDa by MALDI-TOF mass spectrometry, purification of the peptides to near homogeneity by further HPLC, and structural characterization by automated Edman degradation. The effect of synthetic replicates of the active peptides on glucose homeostasis in vivo may be evaluated in mice fed a high fat diet to produce obesity, glucose intolerance, and insulin resistance.

The Potential of Frog Skin-Derived Peptides for Development into Therapeutically-Valuable Immunomodulatory Agents

The aim of this article is to review the immunoregulatory actions of frog skin-derived peptides in order to assess their potential as candidates for immunomodulatory or anti-inflammatory therapy. Frog skin peptides with demonstrable immunomodulatory properties have been isolated from skin secretions of a range of species belonging to the families Alytidae, Ascaphidae, Discoglossidae, Leptodactylidae, Pipidae and Ranidae. Their effects upon production of inflammatory and immunoregulatory cytokines by target cells have been evaluated ex vivo and effects upon cytokine expression and immune cell activity have been studied in vivo by flow cytometry after injection into mice. The naturally-occurring peptides and/or their synthetic analogues show complex and variable actions on the production of proinflammatory (TNF-α, IL-1β, IL-12, IL-23, IL-8, IFN-γ and IL-17), pleiotropic (IL-4 and IL-6) and immunosuppressive (IL-10 and TGF-β) cytokines by peripheral and spleen cells, peritoneal cells and/or isolated macrophages. The effects of frenatin 2.1S include enhancement of the activation state and homing capacity of Th1-type lymphocytes and NK cells in the mouse peritoneal cavity, as well as the promotion of their tumoricidal capacities. Overall, the diverse effects of frog skin-derived peptides on the immune system indicate their potential for development into therapeutic agents.

Identification of the first cathelicidin gene from skin of Chinese giant salamanders Andrias davidianus with its potent antimicrobial activity

Cathelicidins, as effector molecules, play important roles against infections and represent a crucial component of the innate immune system in vertebrates. They are widely studied in mammals, but little is known in amphibians. In the present study, we report the identification and characterization of a novel cathelicidin from Chinese giant salamander Andrias davidianus, which is the first study in Caudata amphibian. The cDNA sequence encodes a predicted 148-amino-acid polypeptide, which composed of a 20-residue signal peptide, a 94-residue conserved cathelin domain and a 34-residue mature peptide. From the multiple sequence alignments and phylogenetic analysis, AdCath shared conserved structure with other orthologs and clustered with other amphibian peptides. The tissue expression profiles revealed AdCath was highly expressed in skin. To study the function of AdCath gene, the AdCath precursor protein and mature peptide were recombinantly expressed and chemical synthesized respectively. The rAdCath protein could bind to LPS in a dose-dependent manner. When the concentrations of rAdCath protein and mature peptide were up to 22 μg/mL, they showed significantly cytotoxicity to human 293T cell lines. The rAdCath protein and synthetic peptide could exhibit antibacterial activities detected by the minimum inhibitory concentrations assay. From the SEM assay, the synthetic mature peptide could destroy the membrane of bacteria and cause loss of membrane integrity. Collectively, these findings characterized the first cathelicidin from A. davidianus, and highlighted its potential antimicrobial activities, indicating its important roles in the skin immune response against different bacteria.

Antimicrobial peptides in frog poisons constitute a molecular toxin delivery system against predators

Animals using toxic peptides and proteins for predation or defense typically depend on specialized morphological structures, like fangs, spines, or a stinger, for effective intoxication. Here we show that amphibian poisons instead incorporate their own molecular system for toxin delivery to attacking predators. Skin-secreted peptides, generally considered part of the amphibian immune system, permeabilize oral epithelial tissue and enable fast access of cosecreted toxins to the predator's bloodstream and organs. This absorption-enhancing system exists in at least three distantly related frog lineages and is likely to be a widespread adaptation, determining the outcome of predator-prey encounters in hundreds of species.

Peptidomic analysis of skin secretions of the Mexican burrowing toad Rhinophrynus dorsalis (Rhinophrynidae): Insight into the origin of host-defense peptides within the Pipidae and characterization of a proline-arginine-rich peptide

The Mexican burrowing toad Rhinophrynus dorsalis is the sole extant representative of the Rhinophrynidae. United in the superfamily Pipoidea, the Rhinophrynidae is considered to be the sister-group to the extant Pipidae which comprises Hymenochirus, Pipa, Pseudhymenochirus and Xenopus. Cationic, α-helical host-defense peptides of the type found in Hymenochirus, Pseudhymenochirus, and Xenopus species (hymenochirins, pseudhymenochirins, magainins, and peptides related to PGLa, XPF, and CPF) were not detected in norepinephrine-stimulated skin secretions of R. dorsalis. Skin secretions of representatives of the genus Pipa also do not contain cationic α-helical host-defense peptides which suggest, as the most parsimonious hypothesis, that the ability to produce such peptides by frogs within the Pipidae family arose in the common ancestor of (Hymenochirus+Pseudhymenochirus)+Xenopus after divergence from the line of evolution leading to extant Pipa species. Peptidomic analysis of the R. dorsalis secretions led to the isolation of rhinophrynin-27, a proline-arginine-rich peptide with the primary structure ELRLPEIARPVPEVLPARLPLPALPRN, together with rhinophrynin-33 containing the C-terminal extension KMAKNQ. Rhinophrynin-27 shows limited structural similarity to the porcine multifunctional peptide PR-39 but it lacks antimicrobial and cytotoxic activities. Like PR-39, the peptide adopts a poly-l-proline helix but some changes in the circular dichroism spectrum were observed in the presence of anionic sodium dodecylsulfate micelles consistent with the stabilization of turn structures.

Expressions of IL-6, TNF-α and NF-κB in the skin of Chinese brown frog (Rana dybowskii)

The cytokine interleukin-6 (IL-6) mediates a wide range of inflammatory and immune responses. Tumor Necrosis Factor α (TNF-α) has a myriad of pro-inflammatory effects on the skin. Nuclear factor κB (NF-κB) is a transcriptional factor that regulates a battery of genes that are critical to immune system. In this study, we investigated the localizations and expression levels of IL-6, TNF-α and NF-κB in the skin of Rana dybowskii during the breeding period and pre-hibernation. Histologically, the skin of Rana dybowskii consisted of epidermis and dermis. Four kinds of cells were identified in the epidermis, while the dermis was composed of homogenous gel, mucous glands and granular glands. IL-6, TNF-α and NF-κB were immunolocalized in the epithelial and glandular cells in both periods. Western blotting showed that IL-6, TNF-α and NF-κB were significantly higher in the pre-hibernation compared to the breeding period. Real- Time PCR revealed that the relative mRNA levels of IL-6 and NB-κB in the pre-hibernation increased significantly compared with the breeding period, while the TNF-α mRNA expression levels were not significantly different between these two periods. These results suggested that IL-6, TNF-α and NF-κB might collectively be involved in the skin immune system of Rana dybowskii during the breeding period and pre-hibernation.

Discovery of two bombinin peptides with antimicrobial and anticancer activities from the skin secretion of Oriental fire-bellied toad, Bombina orientalis

Amphibian skin secretions are known to contain numerous peptides with a large array of biological activities. Bombinins are a group of amphibian-derived peptides with broad spectrum antimicrobial activities that have been only identified from the ancient toad species, Bombina. In this study, we described the identification and characterization of a novel bombinin precursor which encoded a bombinin-like peptide (BLP-7) and a novel bombinin H-type peptide (named as Bombinin H-BO) from the skin secretion of Oriental fire-bellied toad, Bombina orientalis. The primary structures of both mature peptides were determined by combinations of molecular cloning of peptide precursor-encoding cDNAs and mass spectrometry techniques. Secondary structure prediction revealed that both peptides had cationic amphipathic α-helical structural features. The synthetic replicate of BLP-7 displayed more potent antimicrobial activity than Bombinin H-BO against Gram-positive and Gram-negative bacteria and yeast. Also, in vitro antitumour assay showed that both peptides possessed obvious antiproliferative activity on three human hepatoma cells (Hep G2/SK-HEP-1/Huh7) at the non-toxic doses. These results indicate the peptide family of bombinins could be a potential source of drug candidates for anti-infection and anticancer therapy.

OM-LV20, a novel peptide from odorous frog skin, accelerates wound healing in vitro and in vivo

The healing of chronic wounds remains a considerable challenge in clinical trials and imposes severe financial and physiological burdens on patients. Many works are being tried to find ideal clinical promoting wound healing biomaterials. Small bioactive peptides with low cost and easy production, store and transfer become excellent candidates. Here, we identified a novel peptide (named OM-LV20) from skin secretions of odorous frog Odorrana margaretae. The peptide had an amino acid sequence of "LVGKLLKGAVGDVCGLLPIC," contained an intramolecular disulfide bridge at the C-terminus, and was produced by post-translational processing of a 71-residue prepropeptide. Our results showed that OM-LV20 had no direct microbe-killing effects, hemolytic activity, or acute toxicity, but did exhibit weak antioxidant activity. OM-LV20 promoted wound healing against human keratinocytes (HaCaT) and human skin fibroblasts (HSF) in both time- and dose-dependent manners. In addition, it induced the proliferation of HaCaT but not HSF cells. Of note, OM-LV20 showed strong wound healing-promoting activity in a mice model of full-thickness skin wound. Our research indicates the cellular and animal level wound healing potential of OM-LV20, and thus provides a novel bioactive peptide template for the development of wound healing agents and medicine.

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.