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

Determining the pharmacological potential and biological role of linear pseudoscorpion toxins via functional profiling

Arthropod venoms contain bioactive molecules attractive for biomedical applications. However, few of these have been isolated, and only a tiny number has been characterized. Pseudoscorpions are small arachnids whose venom has been largely overlooked. Here, we present the first structural and functional assessment of the checacin toxin family, discovered in the venom of the house pseudoscorpion (Chelifer cancroides). We combined in silico and in vitro analyses to establish their bioactivity profile against microbes and various cell lines. This revealed inhibitory effects against bacteria and fungi. We observed cytotoxicity against specific cell types and effects involving second messengers. Our work provides insight into the biomedical potential and evolution of pseudoscorpion venoms. We propose that plesiotypic checacins evolved to defend the venom gland against infection, whereas apotypic descendants evolved additional functions. Our work highlights the importance of considering small and neglected species in biodiscovery programs.

Recurrent evolution of adhesive defence systems in amphibians by parallel shifts in gene expression

Natural selection can drive organisms to strikingly similar adaptive solutions, but the underlying molecular mechanisms often remain unknown. Several amphibians have independently evolved highly adhesive skin secretions (glues) that support a highly effective antipredator defence mechanism. Here we demonstrate that the glue of the Madagascan tomato frog, Dyscophus guineti, relies on two interacting proteins: a highly derived member of a widespread glycoprotein family and a galectin. Identification of homologous proteins in other amphibians reveals that these proteins attained a function in skin long before glues evolved. Yet, major elevations in their expression, besides structural changes in the glycoprotein (increasing its structural disorder and glycosylation), caused the independent rise of glues in at least two frog lineages. Besides providing a model for the chemical functioning of animal adhesive secretions, our findings highlight how recruiting ancient molecular templates may facilitate the recurrent evolution of functional innovations.

Amphibian host-defense peptides with potential for Type 2 diabetes therapy - an updated review

Investigations conducted since 2018 have identified several host-defense peptides present in frog skin secretions whose properties suggest the possibility of their development into a new class of agent for Type 2 diabetes (T2D) therapy. Studies in vitro have described peptides that (a) stimulate insulin release from BRIN-BD11 clonal β-cells and isolated mouse islets, (b) display β-cell proliferative activity and protect against cytokine-mediated apoptosis and (c) stimulate production of the anti-inflammatory cytokine IL-10 and inhibit production of the pro-inflammatory cytokines TNF-α and IL-1β. Rhinophrynin-27, phylloseptin-3.2TR and temporin F are peptides with therapeutic potential. Studies in vivo carried out in db/db and high fat-fed mice have shown that twice-daily administration of [S4K]CPF-AM1 and [A14K]PGLa-AM1, analogs of peptides first isolated from the octoploid frog Xenopus amieti, over 28 days lowers circulating glucose and HbA1c concentrations, increases insulin sensitivity and improves glucose tolerance and lipid profile. Peptide treatment produced potentially beneficial changes in the expression of skeletal muscle genes involved in insulin signaling and islet genes involved in insulin secretion in these murine models of T2D. Lead compounds uncovered by the study of frog HDPs may provide a basis for the design of new types of agents that can be used, alone or in combination with existing therapies, for the treatment of T2D.

Multi-omics resources for the Australian southern stuttering frog (Mixophyes australis) reveal assorted antimicrobial peptides

The number of genome-level resources for non-model species continues to rapidly expand. However, frog species remain underrepresented, with up to 90% of frog genera having no genomic or transcriptomic data. Here, we assemble the first genomic and transcriptomic resources for the recently described southern stuttering frog (Mixophyes australis). The southern stuttering frog is ground-dwelling, inhabiting naturally vegetated riverbanks in south-eastern Australia. Using PacBio HiFi long-read sequencing and Hi-C scaffolding, we generated a high-quality genome assembly, with a scaffold N50 of 369.3 Mb and 95.1% of the genome contained in twelve scaffolds. Using this assembly, we identified the mitochondrial genome, and assembled six tissue-specific transcriptomes. We also bioinformatically characterised novel sequences of two families of antimicrobial peptides (AMPs) in the southern stuttering frog, the cathelicidins and β-defensins. While traditional peptidomic approaches to peptide discovery have typically identified one or two AMPs in a frog species from skin secretions, our bioinformatic approach discovered 12 cathelicidins and two β-defensins that were expressed in a range of tissues. We investigated the novelty of the peptides and found diverse predicted activities. Our bioinformatic approach highlights the benefits of multi-omics resources in peptide discovery and contributes valuable genomic resources in an under-represented taxon.

Purification, conformational analysis and cytotoxic activities of host-defense peptides from the Tungara frog Engystomops pustulosus (Leptodactylidae; Leiuperinae)

The amphibian family Leptodactylidae is divided into three sub-families: Leiuperinae, Leptodactylinae, and Paratelmatobiinae. Host-defense peptides (HDPs) present in the skins of frogs belonging to the Leptodactylinae have been studied extensively, but information is limited  regarding peptides from Leiuperinae species. Peptidomic analysis of norepinephrine-stimulated skin secretions from the Tungara frog Engystomops pustulosus (Leiuperinae) collected in Trinidad led to the isolation and structural characterization of previously undescribed pustulosin-1 (FWKADVKEIG KKLAAKLAEELAKKLGEQ), [Q28E] pustulosin-1 (pustulosin-2), and pustulosin-3 (DWKETAKELLKKIGAKVAQVISDKLNPAPQ). The primary structures of these peptides do not resemble those of previously described frog skin HDPs. In addition, the secretions contained tigerinin-1EP (GCKTYLIEPPVCT) with structural similarity to the tigerinins previously identified in skin secretions from frogs from the family Dicroglossidae. Pustulosin-1 and -3 adopted extended α-helical conformations in 25% trifluoroethanol-water and in the presence of cell membrane models (sodium dodecylsulfate and dodecylphosphocholine micelles). Pustulosin-1 and -3 displayed cytotoxic activity against a range of human tumor-derived cell lines (A549, MDA-MB-231, and HT29), but their therapeutic potential for development into anti-cancer agents is limited by their comparable cytotoxic activity against non-neoplastic human umbilical vein endothelial cells. The peptides also displayed weak antimicrobial activity against Escherichia coli (MIC = 125 µM) but were inactive against Staphylococcus aureus. Tigerinin-1EP was inactive against both the tumor-derived cells and bacteria.

Identification of an Antimicrobial Peptide from the Venom of the Trinidad Thick-Tailed Scorpion Tityus trinitatis with Potent Activity against ESKAPE Pathogens and Clostridioides difficile

Envenomation by the Trinidad thick-tailed scorpion Tityus trinitatis may result in fatal myocarditis and there is a high incidence of acute pancreatitis among survivors. Peptidomic analysis (reversed-phase HPLC followed by MALDI-TOF mass spectrometry and automated Edman degradation) of T. trinitatis venom led to the isolation and characterization of three peptides with antimicrobial activity. Their primary structures were established asTtAP-1 (FLGSLFSIGSKLLPGVFKLFSRKKQ.NH2), TtAP-2 (IFGMIPGLIGGLISAFK.NH2) and TtAP-3 (FFSLIPSLIGGLVSAIK.NH2). In addition, potassium channel and sodium channel toxins, present in the venom in high abundance, were identified by CID-MS/MS sequence analysis. TtAP-1 was the most potent against a range of clinically relevant Gram-positive and Gram-negative aerobes and against the anaerobe Clostridioides difficile (MIC = 3.1-12.5 µg/mL). At a concentration of 1× MIC, TtAP-1 produced rapid cell death (<15 min against Acinetobacter baumannii and Staphylococcus aureus). The therapeutic potential of TtAP-1 as an anti-infective agent is limited by its high hemolytic activity (LC50 = 18 µg/mL against mouse erythrocytes) but the peptide constitutes a template for the design of analogs that maintain the high bactericidal activity against ESKAPE pathogens but are less toxic to human cells. It is suggested that the antimicrobial peptides in the scorpion venom facilitate the action of the neurotoxins by increasing the membrane permeability of cells from either prey or predator.

Peptidomic analysis of the host-defense peptides in skin secretions of the Amazon River frog Lithobates palmipes (Ranidae)

Skin secretions of certain frog species represent a source of host-defense peptides (HDPs) with therapeutic potential and their primary structures provide insight into taxonomic and phylogenetic relationships. Peptidomic analysis was used to characterize the HDPs in norepinephrine-stimulated skin secretions from the Amazon River frog Lithobates palmipes (Ranidae) collected in Trinidad. A total of ten peptides were purified and identified on the basis of amino acid similarity as belonging to the ranatuerin-2 family (ranatuerin-2PMa, -2PMb, -2PMc, and-2PMd), the brevinin-1 family (brevinin-1PMa, -1PMb, -1PMc and des(8-14)brevinin-1PMa) and the temporin family (temporin-PMa in C-terminally amidated and non-amidated forms). Deletion of the sequence VAAKVLP from brevinin-1PMa (FLPLIAGVAAKVLPKIFCAISKKC) in des[(8-14)brevinin-1PMa resulted in a 10-fold decrease in potency against Staphylococcus aureus (MIC = 31 μM compared with 3 μM) and a > 50-fold decrease in hemolytic activity but potency against Echerichia coli was maintained (MIC = 62.5 μM compared with 50 μM). Temporin-PMa (FLPFLGKLLSGIF.NH₂) inhibited growth of S. aureus (MIC = 16 μM) but the non-amidated form of the peptide lacked antimicrobial activity. Cladistic analysis based upon the primary structures of ranaturerin-2 peptides supports the division of New World frogs of the family Ranidae into the genera Lithobates and Rana. A sister-group relationship between L. palmipes and Warszewitsch's frog Lithobates warszewitschii is indicated within a clade that includes the Tarahumara frog Lithobates tarahumarae. The study has provided further evidence that peptidomic analysis of HDPs in frog skin secretions is a valuable approach to elucidation of the evolutionary history of species within a particular genus.

Mass Spectrometry Differentiation between Rana arvalis Populations Based on Their Skin Peptidome Composition

Skin secretion of amphibians often represents the only weapon of these species against pathogens and predators. Peptides constitute the major portion of active molecules of that weapon and may be treated as potential pharmaceuticals for future generations. The first step of their efficient use involves establishing of their primary structure, i.e., sequencing. De novo sequencing by means of mass spectrometry was applied to Rana arvalis species, collected in the spring 2021 in Central Slovenia (vicinity of Ljubljana). HPLC-ESI-HRMS/MS with Orbitrap instruments was used to establish the skin peptidome of these species and compare it with the earlier identified skin peptidome of the Moscow population of Rana arvalis. Application of CID, HCD, ETD, and EThcD enabled detecting and sequencing 18 peptides; five of them were novel and may be treated as possible biomarkers of the Ljubljana population of Rana arvalis. Interestingly, representatives of two peptide families (temporins and brevinins 2) were not found in the Moscow population. MS³ modes, first of all EThcD, demonstrated their great potential in the de novo sequencing, including extraction of the sequence information from the intact peptides with disulfide cycle (rana box) in their structure and differentiation of isomeric Leu/Ile residues. Thus, all six isomeric residues were reliably distinguished in the novel melittin-related peptide AK-23-1. In addition, another post-translational modification dealing with carbonylation of the N-terminal Gly of novel temporin AVa was established using the MS³ mode. The obtained results demonstrate the efficiency of the use of MS³ tools in proteomics/peptidomics.

Transcriptomic analysis reveals potential candidate pathways and genes involved in toxin biosynthesis in true toads

Synthesized chemical defenses have broadly evolved across countless taxa and are important in shaping evolutionary and ecological interactions within ecosystems. However, the underlying genomic mechanisms by which these organisms synthesize and utilize their toxins are relatively unknown. Herein, we use comparative transcriptomics to uncover potential toxin synthesizing genes and pathways, as well as interspecific patterns of toxin synthesizing genes across ten species of North American true toads (Bufonidae). Upon assembly and annotation of the ten transcriptomes, we explored patterns of relative gene expression and possible protein-protein interactions across the species to determine what genes and/or pathways may be responsible for toxin synthesis. We also tested our transcriptome dataset for signatures of positive selection to reveal how selection may be acting upon potential toxin producing genes. We assembled high quality transcriptomes of the bufonid parotoid gland, a tissue not often investigated in other bufonid related RNAseq studies. We found several genes involved in metabolic and biosynthetic pathways (e.g. steroid biosynthesis, terpenoid backbone biosynthesis, isoquinoline biosynthesis, glucosinolate biosynthesis) that were functionally enriched and/or relatively expressed across the ten focal species that may be involved in the synthesis of alkaloid and steroid toxins, as well as other small metabolic compounds that cause distastefulness in bufonids. We hope that our study lays a foundation for future studies to explore the genomic underpinnings and specific pathways of toxin synthesis in toads, as well as at the macroevolutionary scale across numerous taxa that produce their own defensive toxins.

A review of chemical defense in harlequin toads (Bufonidae: Atelopus)

Toads of the genus Atelopus are chemically defended by a unique combination of endogenously synthesized cardiotoxins (bufadienolides) and neurotoxins which may be sequestered (guanidinium alkaloids). Investigation into Atelopus small-molecule chemical defenses has been primarily concerned with identifying and characterizing various forms of these toxins while largely overlooking their ecological roles and evolutionary implications. In addition to describing the extent of knowledge about Atelopus toxin structures, pharmacology, and biological sources, we review the detection, identification, and quantification methods used in studies of Atelopus toxins to date and conclude that many known toxin profiles are unlikely to be comprehensive because of methodological and sampling limitations. Patterns in existing data suggest that both environmental (toxin availability) and genetic (capacity to synthesize or sequester toxins) factors influence toxin profiles. From an ecological and evolutionary perspective, we summarize the possible selective pressures acting on Atelopus toxicity and toxin profiles, including predation, intraspecies communication, disease, and reproductive status. Ultimately, we intend to provide a basis for future ecological, evolutionary, and biochemical research on Atelopus.

Morphology of the Cutaneous Poison and Mucous Glands in Amphibians with Particular Emphasis on Caecilians (Siphonops annulatus)

Caecilians (order Gymnophiona) are apodan, snake-like amphibians, usually with fossorial habits, constituting one of the most unknown groups of terrestrial vertebrates. As in orders Anura (frogs, tree frogs and toads) and Caudata (salamanders and newts), the caecilian skin is rich in mucous glands, responsible for body lubrication, and poison glands, producing varied toxins used in defence against predators and microorganisms. Whereas in anurans and caudatans skin gland morphology has been well studied, caecilian poison glands remain poorly elucidated. Here we characterised the skin gland morphology of the caecilian Siphonops annulatus, emphasising the poison glands in comparison to those of anurans and salamanders. We showed that S. annulatus glands are similar to those of salamanders, consisting of several syncytial compartments full of granules composed of protein material but showing some differentiated apical compartments containing mucus. An unusual structure resembling a mucous gland is frequently observed in lateral/apical position, apparently connected to the main duct. We conclude that the morphology of skin poison glands in caecilians is more similar to salamander glands when compared to anuran glands that show a much-simplified structure.

Antimicrobial Peptides in Gut Health: A Review

Animal antimicrobial peptides (AMPs), known as broad-spectrum and high-efficiency antibacterial activity, are important effector molecules in innate immune system. AMPs not only have antimicrobial, antiviral and antitumor effects but also exhibit important effects in vivo, such as anti-inflammatory response, recruiting immune cells, promoting epithelial damage repair, and promoting phagocytosis of bacteria. However, research on the application of AMPs is incomplete and controversial. This review mainly introduces the classification of AMPs, biological functions, as well as the mechanisms of action, expression rules, and nutrition regulation from three perspectives, aiming to provide important information for the application of AMPs.

Ecological Risks Due to Immunotoxicological Effects on Aquatic Organisms

The immunotoxic effects of some anthropogenic pollutants on aquatic organisms are among the causes of concern over the presence of these pollutants in the marine environment. The immune system is part of an organism's biological defense necessarily for homeostasis. Thus, the immunotoxicological impacts on aquatic organisms are important to understand the effects of pollutant chemicals in the aquatic ecosystem. When aquatic organisms are exposed to pollutant chemicals with immunotoxicity, it results in poor health. In addition, aquatic organisms are exposed to pathogenic bacteria, viruses, parasites, and fungi. Exposure to pollutant chemicals has reportedly caused aquatic organisms to show various immunotoxic symptoms such as histological changes of lymphoid tissue, changes of immune functionality and the distribution of immune cells, and changes in the resistance of organisms to infection by pathogens. Alterations of immune systems by contaminants can therefore lead to the deaths of individual organisms, increase the general risk of infections by pathogens, and probably decrease the populations of some species. This review introduced the immunotoxicological impact of pollutant chemicals in aquatic organisms, including invertebrates, fish, amphibians, and marine mammals; described typical biomarkers used in aquatic immunotoxicological studies; and then, discussed the current issues on ecological risk assessment and how to address ecological risk assessment through immunotoxicology. Moreover, the usefulness of the population growth rate to estimate the immunotoxicological impact of pollution chemicals was proposed.

Aggregation and Its Influence on the Bioactivities of a Novel Antimicrobial Peptide, Temporin-PF, and Its Analogues

Temporin is an antimicrobial peptide (AMP) family discovered in the skin secretion of ranid frog that has become a promising alternative for conventional antibiotic therapy. Herein, a novel temporin peptide, Temporin-PF (TPF), was successfully identified from Pelophylax fukienensis. It exhibited potent activity against Gram-positive bacteria, but no effect on Gram-negative bacteria. Additionally, TPF exhibited aggregation effects in different solutions. Three analogs were further designed to study the relationship between the aggregation patterns and bioactivities, and the MD simulation was performed for revealing the pattern of the peptide assembly. As the results showed, all peptides were able to aggregate in the standard culture media and salt solutions, especially CaCl2 and MgCl2 buffers, where the aggregation was affected by the concentration of the salts. MD simulation reported that all peptides were able to form oligomers. The parent peptide assembly depended on the hydrophobic interaction via the residues in the middle domain of the sequence. However, the substitution of Trp/D-Trp resulted in an enhanced inter-peptide interaction in the zipper-like domain and eliminated overall biological activities. Our study suggested that introducing aromaticity at the zipper-like domain for temporin may not improve the bioactivities, which might be related to the formation of aggregates via the inter-peptide contacts at the zipper-like motif domain, and it could reduce the binding affinity to the lipid membrane of microorganisms.

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

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

Comparative research on morphology and mechanical property of integument of Rana dybowskii, Xenopus laevis and Ambystoma mexicanum

Amphibians' integument is a multifunctional organ offering protection from the exterior surroundings and facilitating the physiological change of gas, water and salts with the environment, which is a natural biomaterial with multifunctional features. Interspecies comparison of biomechanical characters and microstructure possibly related to them were performed on the integument of three species of amphibians, two anurans(Rana dybowskii and Xenopus laevis) and one urodeles(Ambystoma mexicanum) using tensile testing and morphological characterization. It was found that the integument of Rana dybowskii and Xenopus laevis was covered by polygonal epidermal cells, while the trunk surface of Ambystoma mexicanum presented irregular microstructure with the lack of keratinization. The integument of Rana dybowskii and Xenopus laevis exhibited good performance on stiffness and strength, which showed quite high mean elastic modulus, 931MPa and 1048MPa,respectively.

Rapid toxin sequestration modifies poison frog physiology

Poison frogs sequester chemical defenses from their diet of leaf litter arthropods for defense against predation. Little is known about the physiological adaptations that confer this unusual bioaccumulation ability. We conducted an alkaloid-feeding experiment with the Diablito poison frog (Oophaga sylvatica) to determine how quickly alkaloids are accumulated and how toxins modify frog physiology using quantitative proteomics. Diablito frogs rapidly accumulated the alkaloid decahydroquinoline within 4 days, and dietary alkaloid exposure altered protein abundance in the intestines, liver and skin. Many proteins that increased in abundance with decahydroquinoline accumulation are plasma glycoproteins, including the complement system and the toxin-binding protein saxiphilin. Other protein classes that change in abundance with decahydroquinoline accumulation are membrane proteins involved in small molecule transport and metabolism. Overall, this work shows that poison frogs can rapidly accumulate alkaloids, which alter carrier protein abundance, initiate an immune response, and alter small molecule transport and metabolism dynamics across tissues.

Identification of New Ocellatin Antimicrobial Peptides by cDNA Precursor Cloning in the Frame of This Family of Intriguing Peptides

Ocellatins are a family of antimicrobial peptides found exclusively in the Leptodactylus genus. To date, 10 species have been studied and more than 23 peptides described. Here we report the sequences of five new peptides from the skin of the frog Leptodactylus latrans (Anura: Leptodactylidae) determined by cDNA cloning of the complete prepro-peptide structures. The mature peptides were characterized with in silico tools and compared with those previously described. With 21 amino acid residues, this new set of peptides not previously described in the Leptodactylus genus share between 100 and 76.2% similarity to ocellatin antimicrobial peptides. These novel peptides are cationic and their three-dimensional (3D) structure holds the highly conserved residues G¹, D⁴, K⁷, and K¹¹ and a high theoretical amphipathic α-helix content. Furthermore, in silico analyses of these new peptides predicted antimicrobial activity. This study is framed in the context of previous work published about ocellatins, and therefore, provides a review of this intriguing family of peptides.

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

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

Proteo-Transcriptomic Analysis Identifies Potential Novel Toxins Secreted by the Predatory, Prey-Piercing Ribbon Worm Amphiporus lactifloreus

Nemerteans (ribbon worms) employ toxins to subdue their prey, but research thus far has focused on the small-molecule components of mucus secretions and few protein toxins have been characterized. We carried out a preliminary proteotranscriptomic analysis of putative toxins produced by the hoplonemertean Amphiporus lactifloreus (Hoplonemertea, Amphiporidae). No variants were found of known nemertean-specific toxin proteins (neurotoxins, cytotoxins, parbolysins or nemertides) but several toxin-like transcripts were discovered, expressed strongly in the proboscis, including putative metalloproteinases and sequences resembling sea anemone actitoxins, crown-of-thorn sea star plancitoxins, and multiple classes of inhibitor cystine knot/knottin family proteins. Some of these products were also directly identified in the mucus proteome, supporting their preliminary identification as secreted toxin components. Two new nemertean-typical toxin candidates could be described and were named U-nemertotoxin-1 and U-nemertotoxin-2. Our findings provide insight into the largely overlooked venom system of nemerteans and support a hypothesis in which the nemertean proboscis evolved in several steps from a flesh-melting organ in scavenging nemerteans to a flesh-melting and toxin-secreting venom apparatus in hunting hoplonemerteans.

In-situ sulfuration of Cu-based metal-organic framework for rapid near-infrared light sterilization

Some new kinds of antibiotics-free antibacterial agents are required to deal with bacterial infections due to the occurrence of drug-resistance. In this work, Cu-based metal-organic framework (HKUST-1) embedded with CuS NPs were fabricated via a simple in-situ sulfuration process. The synthesized MOFs exhibited an highly effective disinfection efficacy of 99.70 % and 99.80 % against Staphylococcus aureus and Escherichia coli within 20 min irradiation of near-infrared (NIR) light, respectively, which was ascribed to the cooperative effects of photodynamic and photothermal effects of the composites. A certain amount of Cu²⁺ ions of the MOFs were reacted to form CuS NPs, which endowed this composite with outstanding photocatalytic and photothermal performance during NIR light irradiation. Moreover, HKUST-1 that composed of low toxic organic ligand 1,3,5-benzenetricarboxylic acid (H₃BTC) coordinating copper ions could be a controllable carrier that imposed certain constraint on the NPs. Hence, these CuS@HKUST-1 would be a promising bioplatform for rapid bacteria-killing.

Distribution of major toxins in Rhinella marina parotoid macroglands using Desorption-Electrospray-Ionization mass spectrometry imaging (DESI-MSI)

Amphibian cutaneous glands secrete toxins used in different vital functions including passive defense. Through Desorption Electrospray Ionization-Imaging we analyzed the distribution of the major toxins of the toad Rhinella marina parotoid macroglands. Alkaloids and steroids showed characteristic distribution and intensity within the glands and were also present at lower levels on the skin surface. A comprehensive overview of toxins distribution in toads’ skin might help to understand their full biological role within the amphibians.

•

Amphibians possess granular glands associated to chemical defense.

•

Actual tissue distribution of toads' toxins remains unknown.

•

We have pinpointed some toad toxins in the parotoid macroglands.

•

Alkaloids are distributed throughout the gland; whereas steroids are peripheral.

•

Toxins are stored within the macrogland, presenting large concentration variation.

A multifunctional plasmonic chip for bacteria capture, imaging, detection, and in situ elimination for wound therapy

A multifunctional plasmonic gold chip has been constructed for early diagnosis and highly effective killing of bacteria, which is critical for human health. The chip features high bacterial capture efficiency, plasmon-enhanced fluorescence (PEF) and surface-enhanced Raman scattering (SERS) and can act as a highly sensitive sensor for dual-mode bacteria imaging and detection (down to 102 CFU mL-1) with good reliability and accuracy. The developed assay can distinguish Gram-positive S. aureus bacteria from Gram-negative E. coli bacteria, providing valuable information for therapy. Importantly, the chip presents excellent photothermal antibacterial activity (98%) and can inactivate both Gram-positive and Gram-negative bacteria in situ. Furthermore, the chip was used to effectively promote the wound healing process in bacteria infected mice in vivo, showing great potential for antibacterial applications.

Improving CRISPR-Cas9 Genome Editing Efficiency by Fusion with Chromatin-Modulating Peptides

Bacterial-derived CRISPR-Cas9 nucleases have become a common tool in genome engineering. However, the editing efficiency by even the best-crafted Cas9 nucleases varies considerably with different genomic sites, and efforts to explore the vast natural Cas9 diversity have often met with mixed or little success. Here, we show that modification of the widely used Streptococcus pyogenes Cas9 by fusion with chromatin-modulating peptides (CMPs), derived from high mobility group proteins HMGN1 and HMGB1, histone H1, and chromatin remodeling complexes, improves its activity by up to several fold, particularly on refractory target sites. We further show that this CMP fusion strategy (termed CRISPR-chrom) is also effective in improving the activities of smaller Cas9 nucleases from Streptococcus pasteurianus and Campylobacter jejuni, as well as four newly characterized Cas9 orthologs from Bacillus smithii, Lactobacillus rhamnosus, Mycoplasma canis, and Parasutterella excrementihominis. Our findings suggest that this CRISPR-chrom strategy can be used to improve established Cas9 nucleases and facilitate exploration of novel Cas9 orthologs for genome modification.

On-demand storage and release of antimicrobial peptides using Pandora's box-like nanotubes gated with a bacterial infection-responsive polymer

Background: Localized delivery of antimicrobial agents such as antimicrobial peptides (AMPs) by a biomaterial should be on-demand. Namely, AMPs should be latent and biocompatible in the absence of bacterial infection, but released in an amount enough to kill bacteria immediately in response to bacterial infection. Methods: To achieve the unmet goal of such on-demand delivery, here we turned a titanium implant with titania nanotubes (Ti-NTs) into a Pandora's box. The box was loaded with AMPs (HHC36 peptides, with a sequence of KRWWKWWRR) inside the nanotubes and "closed" (surface-modified) with a pH-responsive molecular gate, poly(methacrylic acid) (PMAA), which swelled under normal physiological conditions (pH 7.4) but collapsed under bacterial infection (pH ≤ 6.0). Thus, the PMAA-gated Ti-NTs behaved just like a Pandora's box. The box retarded the burst release of AMPs under physiological conditions because the gate swelled to block the nanotubes opening. However, it was opened to release AMPs to kill bacteria immediately when bacterial infection occurred to lowering the pH (and thus made the gate collapse). Results: We demonstrated such smart excellent bactericidal activity against a panel of four clinically important bacteria, including Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus. In addition, this box was biocompatible and could promote the osteogenic differentiation of human mesenchymal stem cells. Both in vitro and in vivo studies confirmed the smart "on-demand" bactericidal activity of the Pandora's box. The molecularly gated Pandora's box design represents a new strategy in smart drug delivery.

The Mechanism of Membrane Permeabilization by Peptides: Still an Enigma

Peptide-induced permeabilization of lipid vesicles has been measured for decades and has provided many insights into the sequence-structure-function relationships of membrane-active peptides. However, researchers in the field have noted that many experiments show transient permeabilization, in which a burst of leakage occurs immediately after peptide addition, followed by a slowdown or cessation of leakage before all contents have been released. This widely observed, but rarely studied, phenomenon is not explained by standard equilibrium pore models that are commonly invoked in both experimental and computational studies. Here we discuss observations of transient permeabilization, and we outline a pathway towards understanding this enigmatic phenomenon.

Protein identification from the parotoid macrogland secretion of Duttaphrynus melanostictus

Background:

Bufonid parotoid macrogland secretion contains several low molecular mass molecules, such as alkaloids and steroids. Nevertheless, its protein content is poorly understood. Herein, we applied a sample preparation methodology that allows the analysis of viscous matrices in order to examine its proteins.

Methods:

Duttaphrynus melanostictus parotoid macrogland secretion was submitted to ion-exchange batch sample preparation, yielding two fractions: salt-displaced fraction and acid-displaced fraction. Each sample was then fractionated by anionic-exchange chromatography, followed by in-solution proteomic analysis.

Results:

Forty-two proteins could be identified, such as acyl-CoA-binding protein, alcohol dehydrogenase, calmodulin, galectin and histone. Moreover, de novo analyses yielded 153 peptides, whereas BLAST analyses corroborated some of the proteomic-identified proteins. Furthermore, the de novo peptide analyses indicate the presence of proteins related to apoptosis, cellular structure, catalysis and transport processes.

Conclusions:

Proper sample preparation allowed the proteomic and de novo identification of different proteins in the D. melanostictus parotoid macrogland secretion. These results may increase the knowledge about the universe of molecules that compose amphibian skin secretion, as well as to understand their biological/physiological role in the granular gland.

Spontaneous and Stress-Induced Pore Formation in Membranes: Theory, Experiments and Simulations

The large plasticity, dynamics and adaptability of biological membranes allow different modes of intrinsic and inducible permeability. These phenomena are of physiological importance for a number of natural functions related to cell death and can also be manipulated artificially for practical purposes like gene transfer, drug delivery, prevention of infections or anticancer therapy. For these advances to develop in a controllable and specific way, we need a sufficient understanding of the membrane permeability phenomena. Since the formulation of early concepts of pore formation, there has been an enormous effort to describe membrane permeability by using theory, simulations and experiments. A major breakthrough has come recently through theoretical developments that allow building continuous trajectories of pore formation both in the absence and presence of stress conditions. The new model provides a coherent quantitative view of membrane permeabilization, useful to test the impact of known lipid properties, make predictions and postulate specific pore intermediates that can be studied by simulations. For example, this theory predicts unprecedented dependencies of the line tension on the pore radius and on applied lateral tension which explain previous puzzling results. In parallel, important concepts have also come from molecular dynamics simulations, of which the role of water for membrane permeabilization is of special interest. These advances open new challenges and perspectives for future progress in the study of membrane permeability, as experiments and simulations will need to test the theoretical predictions, while theory achieves new refinements that provide a physical ground for observations.

Detecting the structural assembly pathway of human antimicrobial peptide pores at single-channel level

The pore-forming structures of an anionic human antimicrobial peptide dermcidin (DCD) in a membrane environment has not been demonstrated previously. Using single-channel electrical recordings, we characterized the structural and functional properties of the DCD peptide channel in lipid membranes. We show that a 48-residue, 8 nm long anionic DCD-1L peptide is folded in the right conformation in sodium dodecyl sulfate (SDS) that spontaneously inserts into lipid bilayers to form well-defined channels. However, the DCD-1L peptides are not properly folded in n-dodecyl-β-d-maltoside (DDM), resulting in unstable channels suggesting the significance of specific detergent in stable channel formation. Furthermore, a 25-residue cationic DCD SSL-25 peptide formed channels both in SDS and DDM micelles as the length of the peptide matches with the thickness of the membrane. Finally, we quantified the permeation of small molecules through the DCD channels in liposome assays. Accordingly, we propose a molecular model demonstrating the structural self-assembly of the DCD channels in the membrane. We suggest that an understanding of the mechanism of action of DCD peptides at single-channel resolution will lead to developing peptide-based therapeutics.

Isolation and Identification of Alkaloids from Poisons of Fire Salamanders ( Salamandra salamandra)

Fire salamanders ( Salamandra salamandra) are conspicuously colored amphibians secreting a skin poison that contains unique steroid alkaloids such as samandarine (1) and samadarone (2), exhibiting toxic as well as antimicrobial activities. Because of their antipredatory and anti-infectious functions, alkaloids from Salamandra poison are of interest with regard to the threat that the lethal fungus Batrachochytrium salamandrivorans ( Bsal) poses to salamanders. Nevertheless, reliable data on the biological activity of Salamandra alkaloids are scarce, in part due to the difficulty to obtain and study those substances. Thus, isolation of pure salamander alkaloids is an important task that might directly contribute to the understanding of Bsal infections. Here we present a noninvasive isolation procedure for samandarine (1) and O-acetylsamandarine (3), as well as for two new alkaloids, O-3-hydroxybutanoylsamandarine (4) and samanone (6), using HPLC. For the first time, high-field NMR data are presented for these alkaloids. Analysis using GC/MS and ESI⁺-MS, provided important information on the structural variability of these salamander alkaloids.

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.

Mechanistic Landscape of Membrane-Permeabilizing Peptides

Membrane permeabilizing peptides (MPPs) are as ubiquitous as the lipid bilayer membranes they act upon. Produced by all forms of life, most membrane permeabilizing peptides are used offensively or defensively against the membranes of other organisms. Just as nature has found many uses for them, translational scientists have worked for decades to design or optimize membrane permeabilizing peptides for applications in the laboratory and in the clinic ranging from antibacterial and antiviral therapy and prophylaxis to anticancer therapeutics and drug delivery. Here, we review the field of membrane permeabilizing peptides. We discuss the diversity of their sources and structures, the systems and methods used to measure their activities, and the behaviors that are observed. We discuss the fact that "mechanism" is not a discrete or a static entity for an MPP but rather the result of a heterogeneous and dynamic ensemble of structural states that vary in response to many different experimental conditions. This has led to an almost complete lack of discrete three-dimensional active structures among the thousands of known MPPs and a lack of useful or predictive sequence-structure-function relationship rules. Ultimately, we discuss how it may be more useful to think of membrane permeabilizing peptides mechanisms as broad regions of a mechanistic landscape rather than discrete molecular processes.

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.

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.

OA-GL21, a novel bioactive peptide from Odorrana andersonii, accelerated the healing of skin wounds

Nowadays, the number of chronic trauma cases caused by a variety of factors such as the world's population-ageing and chronic diseases is increasing steadily, and thus effective treatment for chronic wounds has become a severe clinical challenge, which also burdens the patient both physically and financially. Therefore, it is urgent to develop new drugs to accelerate the healing of wounds. Bioactive peptides, which are relatively low cost, easy to produce, store and transport, have become an excellent choice. In this research, we identified a novel peptide OA-GL21, with an amino acid sequence of 'GLLSGHYGRVVSTQSGHYGRG', from the skin secretions of Odorrana andersonii Our results showed that OA-GL21 exerted the ability to promote wound healing of human keratinocytes (HaCaT) and human fibroblasts in a dose- and time-denpendent manner. However, OA-GL21 had no significant effect on the proliferation of these two cells. Significantly, OA-GL21 showed obvious ability to promote wound healing in the full-thickness skin wound model in dose- and scar-free manners. Further studies showed that OA-GL21 had no direct antibacterial, hemolytic, and acute toxic activity; it had weak antioxidant activities but high stability. In conclusion, this research proved the promoting effects of OA-GL21 on cellular and animal wounds, and thus provided a new peptide template for the development of wound-repairing drugs.

Cleavage of Peptides from Amphibian Skin Revealed by Combining Analysis of Gland Secretion and in Situ MALDI Imaging Mass Spectrometry

Peptides from skin secretions of amphibians are considered important components of their immune system and also play a relevant role in their defense mechanism against predators. Herein, by using mass spectrometry (MS), we characterize the sequence of 13 peptides from the gland secretion of the hylid tree frog, Boana punctata. Using in situ matrix-assisted laser desorption ionization imaging MS of a transverse section of the skin tissue, we show that some peptides are stored as longer molecules that are cleaved after being secreted, whereas others do not undergo any modification. Sequence comparison with peptides from other Boana species and analysis of the three-dimensional theoretical structure indicate that this cleavage depends on both the presence of a specific sequence motif and the secondary structure. The fact that peptides undergo a rapid cleavage upon secretion suggests that stored and secreted peptides may have distinct roles for anuran survival, including defense against pathogens and predators.

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.

A Bowman-Birk type chymotrypsin inhibitor peptide from the amphibian, Hylarana erythraea

The first amphibian skin secretion-derived Bowman-Birk type chymotrypsin inhibitor is described here from the Asian green frog, Hylarana erythraea, and was identified by use of molecular cloning and tandem mass spectrometric amino acid sequencing. It was named Hylarana erythraea chymotrypsin inhibitor (HECI) and in addition to inhibition of chymotrypsin (Ki = 3.92 ± 0.35 μM), the peptide also inhibited the 20 S proteasome (Ki = 8.55 ± 1.84  μM). Additionally, an analogue of HECI, named K⁹-HECI, in which Phe⁹ was substituted by Lys⁹ at the P1 position, was functional as a trypsin inhibitor. Both peptides exhibited anti-proliferation activity against the human cancer cell lines, H157, PC-3 and MCF-7, up to a concentration of 1 mM and possessed a low degree of cytotoxicity on normal cells, HMEC-1. However, HECI exhibited higher anti-proliferative potency against H157. The results indicate that HECI, inhibiting chymotryptic-like activity of proteasome, could provide new insights in treatment of lung cancer.