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

Insights into caudate amphibian skin secretions with a focus on the chemistry and bioactivity of derived peptides

Amphibians are well-known for their ability to produce and secrete a mixture of bioactive substances in specialized skin glands for the purpose of antibiotic self-protection and defense against predators. Some of these secretions contain various small molecules, such as the highly toxic batrachotoxin, tetrodotoxin, and samandarine. For some time, the presence of peptides in amphibian skin secretions has attracted researchers, consisting of a diverse collection of - to the current state of knowledge - three to 104 amino acid long sequences. From these more than 2000 peptides many are known to exert antimicrobial effects. In addition, there are some reports on amphibian skin peptides that can promote wound healing, regulate immunoreactions, and may serve as antiparasitic and antioxidative substances. So far, the focus has mainly been on skin peptides from frogs and toads (Anura), eclipsing the research on skin peptides of the ca. 700 salamanders and newts (Caudata). Just recently, several novel observations dealing with caudate peptides and their structure-function relationships were reported. This review focuses on the chemistry and bioactivity of caudate amphibian skin peptides and their potential as novel agents for clinical applications.

Andrias Davidianus Mucus-Based Bioadhesive with Enhanced Adhesion and Wound Healing Properties

The skin secretion of Andrias davidianus (SSAD) is a novel biological adhesive raw material under development. This material exhibits robust adhesion while maintaining the flexibility of the wound. It also has the potential for large-scale production, making it promising for practical application explore. Hence, in-depth research on methods to fine-tune SSAD properties is of great importance to promote its practical applications. Herein, we aim to enhance the adhesive and healing properties of SSAD by incorporating functional components. To achieve this goal, we selected 3,4-dihydroxy-l-phenylalanine and vaccarin as the functional components and mixed them with SSAD, resulting in a new bioadhesive, namely, a formulation termed "enhanced SSAD" (ESSAD). We found that the ESSAD exhibited superior adhesive properties, and its adhesive strength was improved compared with the SSAD. Moreover, ESSAD demonstrated a remarkable ability to promote wound healing. This study presents an SSAD-based bioadhesive formulation with enhanced properties, affirming the feasibility of developing SSAD-based adhesive materials with excellent performance and providing new evidence for the application of SSAD. This study also aims to show that SSAD can be mixed with other substances, and addition of effective components to SSAD can be studied to further adjust or improve its performance.

In Vitro Investigation of the Antibacterial Activity of Salamander Skin Peptides

Given the current and future costs of antibiotic-resistant bacteria to human health and economic productivity, there is an urgent need to develop new antimicrobial compounds. Antimicrobial peptides are a promising alternative to conventional antibiotics and other antimicrobials. Amphibian skin is a rich source of bioactive compounds, but the antibacterial properties of salamander skin peptides have been neglected. Here, we examined the in vitro ability of skin peptides from 9 species of salamander representing 6 salamander families to inhibit the growth of ESKAPE pathogens, which are bacteria that have developed resistance to conventional antibiotics. We also examined whether the skin peptides caused lysis of human red blood cells. Skin peptides from Amphiuma tridactylum had the greatest antimicrobial properties, completely inhibiting the growth of all bacterial strains except for Enterococcus faecium. Likewise, skin peptides from Cryptobranchus alleganiensis completely inhibited the growth of several of the bacterial strains. In contrast, skin peptide mixtures from Ambystoma maculatum, Desmognathus fuscus, Eurycea bislineata, E. longicauda, Necturus beyeri, N. maculosus, and Siren intermedia did not completely inhibit bacterial growth even at the highest concentrations. Finally, none of the skin peptide mixtures caused lysis of human red blood cells. Together, we demonstrate that salamander skin produces peptides with potent antibacterial properties. It remains to elucidate the peptide sequences and their antibacterial mechanisms.

Antibacterial Indole Diketopiperazine Alkaloids from the Deep-Sea Cold Seep-Derived Fungus Aspergillus chevalieri

A large body of fungal secondary metabolites has been discovered to exhibit potent antibacterial activities with distinctive mechanisms and has the potential to be an untapped resource for drug discovery. Here, we describe the isolation and characterization of five new antibacterial indole diketopiperazine alkaloids, namely 24,25-dihydroxyvariecolorin G (1), 25-hydroxyrubrumazine B (2), 22-chloro-25-hydroxyrubrumazine B (3), 25-hydroxyvariecolorin F (4), and 27-epi-aspechinulin D (5), along with the known analogue neoechinulin B (6) from a fungal strain of deep-sea cold seep-derived Aspergillus chevalieri. Among these compounds, 3 and 4 represented a class of infrequently occurring fungal chlorinated natural products. Compounds 1-6 showed inhibitory activities against several pathogenic bacteria with MIC values ranging from 4 to 32 μg/mL. It was revealed that compound 6 could induce structural damage to the Aeromonas hydrophila cells based on the observation by scanning electron microscopy (SEM), which led to the bacteriolysis and death of A. hydrophila, suggesting that neoechinulin B (6) might be a potential alternative to novel antibiotics development.

Efficacy of natural antimicrobial peptides versus peptidomimetic analogues: a systematic review

Aims: This systematic review was carried out to determine whether synthetic peptidomimetics exhibit significant advantages over antimicrobial peptides in terms of in vitro potency. Structural features - molecular weight, charge and length - were examined for correlations with activity. Methods: Original research articles reporting minimum inhibitory concentration  values against Escherichia coli, indexed until 31 December 2020, were searched in PubMed/ScienceDirect/Google Scholar and evaluated using mixed-effects models. Results: In vitro antimicrobial activity of peptidomimetics resembled that of antimicrobial peptides. Net charge significantly affected minimum inhibitory concentration values (p < 0.001) with a trend of 4.6% decrease for increments in charge by +1. Conclusion: AMPs and antibacterial peptidomimetics exhibit similar potencies, providing an opportunity to exploit the advantageous stability and bioavailability typically associated with peptidomimetics.

Ramosin: The First Antibacterial Peptide Identified on Bolitoglossa ramosi Colombian Salamander

The discovery and improvements of antimicrobial peptides (AMPs) have become an alternative to conventional antibiotics. They are usually small and heat-stable peptides, exhibiting inhibitory activity against Gram-negative and Gram-positive bacteria. In this way, studies on broad-spectrum AMPs found in amphibians with the remarkable capability to regenerate a wide array of tissues are of particular interest in the search for new strategies to treat multidrug-resistant bacterial strains. In this work, the use of bioinformatic approaches such as sequence alignment with Fasta36 and prediction of antimicrobial activity allowed the identification of the Ramosin peptide from the de novo assembled transcriptome of the plethodontid salamander Bolitoglossa ramosi obtained from post-amputation of the upper limb tissue, heart, and intestine samples. BLAST analysis revealed that the Ramosin peptide sequence is unique in Bolitoglossa ramosi. The peptide was chemically synthesized, and physicochemical properties were characterized. Furthermore, the in vitro antimicrobial activity against relevant Gram-positive and Gram-negative human pathogenic bacteria was demonstrated. Finally, no effect against eukaryotic cells or human red blood cells was evidenced. This is the first antibacterial peptide identified from a Colombian endemic salamander with interesting antimicrobial properties and no hemolytic activity.

A non-bactericidal cathelicidin provides prophylactic efficacy against bacterial infection by driving phagocyte influx

The roles of bactericidal cathelicidins against bacterial infection have been extensively studied. However, the antibacterial property and mechanism of action of non-bactericidal cathelicidins are rarely known. Herein, a novel naturally occurring cathelicidin (PopuCATH) from tree frog (Polypedates puerensis) did not show any direct anti-bacterial activity in vitro. Intriguingly, intraperitoneal injection of PopuCATH before bacterial inoculation significantly reduced the bacterial load in tree frogs and mice, and reduced the inflammatory response induced by bacterial inoculation in mice. PopuCATH pretreatment also increased the survival rates of septic mice induced by a lethal dose of bacterial inoculation or cecal ligation and puncture (CLP). Intraperitoneal injection of PopuCATH significantly drove the leukocyte influx in both frogs and mice. In mice, PopuCATH rapidly drove neutrophil, monocyte/macrophage influx in mouse abdominal cavity and peripheral blood with a negligible impact on T and B lymphocytes, and neutrophils, monocytes/macrophages, but not T and B lymphocytes, were required for the preventive efficacy of PopuCATH. PopuCATH did not directly act as chemoattractant for phagocytes, but PopuCATH obviously drove phagocyte migration when it was cultured with macrophages. PopuCATH significantly elicited chemokine/cytokine production in macrophages through activating p38/ERK mitogen-activated protein kinases (MAPKs) and NF-κB p65. PopuCATH markedly enhanced neutrophil phagocytosis via promoting the release of neutrophil extracellular traps (NETs). Additionally, PopuCATH showed low side effects both in vitro and in vivo. Collectively, PopuCATH acts as a host-based immune defense regulator that provides prophylactic efficacy against bacterial infection without direct antimicrobial effects. Our findings reveal a non-bactericidal cathelicidin which possesses unique anti-bacterial action, and highlight the potential of PopuCATH to prevent bacterial infection.

A novel anionic cathelicidin lacking direct antimicrobial activity but with potent anti-inflammatory and wound healing activities from the salamander Tylototriton kweichowensis

Cathelicidin is a family of antimicrobial peptides (AMPs) existing in vertebrates, which play multiple functions in host responses against environmental stresses. All cathelicidins identified to date are cationic, no anionic member with net negative charges has been reported. In the present study, a novel anionic cathelicidin (TK-CATH) with a net charge of -3 was identified from the skin of the salamander, T. kweichowensis. Unlike most other cathelicidin members, it didn't exhibit direct antimicrobial activity. However, it demonstrated strong anti-inflammatory activity. It effectively inhibited the LPS-induced pro-inflammatory cytokine gene expression and protein production in amphibian leukocytes and mouse macrophages by inhibiting the LPS-activated mitogen-activated protein kinase (MAPK) signaling pathways. Besides, TK-CATH showed potent wound healing activity. It could effectively induce the production of several cytokines, chemokines and growth factors relating to wound healing, promote the motility and proliferation of keratinocytes, and accelerate the skin wound healing in a mouse full-thickness wound model. These results imply that TK-CATH participates in both the inflammatory phase and new tissue formation phase of wound repair process. Meanwhile, TK-CATH exhibited weak but effective free radical scavenging activity and low cytotoxicity. All the results above indicate that TK-CATH is a multifunctional peptide in the skin of the salamander T. kweichowensis. It may play important roles in host immune responses against bacterial infection and skin wound repair.

Characterization and functional analysis of cathelicidin-MH, a novel frog-derived peptide with anti-septicemic properties

Antimicrobial peptides form part of the innate immune response and play a vital role in host defense against pathogens. Here we report a new antimicrobial peptide belonging to the cathelicidin family, cathelicidin-MH (cath-MH), from the skin of Microhyla heymonsivogt frog. Cath-MH has a single α-helical structure in membrane-mimetic environments and is antimicrobial against fungi and bacteria, especially Gram-negative bacteria. In contrast to other cathelicidins, cath-MH suppresses coagulation by affecting the enzymatic activities of tissue plasminogen activator, plasmin, β-tryptase, elastase, thrombin, and chymase. Cath-MH protects against lipopolysaccharide (LPS)- and cecal ligation and puncture-induced sepsis, effectively ameliorating multiorgan pathology and inflammatory cytokine through its antimicrobial, LPS-neutralizing, coagulation suppressing effects as well as suppression of MAPK signaling. Taken together, these data suggest that cath-MH is an attractive candidate therapeutic agent for the treatment of septic shock.

Identification and characteristics of a novel cecropin from the armyworm, Mythimna separata

BACKGROUND

The recent emergence of antibiotic-resistant strains of bacteria has increased the need to develop effective alternatives to antibiotics. Antimicrobial peptides have been considered as a promising product with several advantages.

RESULTS

In this present study, we identified a novel cecropin from the armyworm, Mythimna separata (armyworm cecropin 1, AC-1) by transcriptome sequencing and multi-sequence alignment analysis. The AC-1 precursor comprised 63 amino acid residues, containing a conserved cleavage site of the signal peptide, Ala₂₃-Pro₂₄, while the mature AC-1 included 39 amino acid residues. Chemically synthesized AC-1 exhibited low hemolytic activity against chicken red blood cells, low cytotoxicity against swine testis cells, and effective antimicrobial activity against Salmonella, Escherichia coli, Klebsiella pneumonia, and Pseudomonas aeruginosa. Its antimicrobial activity against Salmonella remained after incubation for 1 h at 100 °C or in 250 mM NaCl, KCl, or MgCl₂ solution, implying good thermal- and salt-resistant stabilities. The bactericidal effect of AC-1 on E. coli gradually increased with increasing AC-1 concentration, resulting in deformation, severe edema, cytolysis, cell membrane damage, and reducing intracellular electron density. Additionally, recombinant AC-1 protein expressed in E. coli was digested by enterokinase protease to obtain AC-1, which showed similar antimicrobial activity against E. coli to chemically synthesized AC-1.

CONCLUSIONS

This study identified a novel antimicrobial peptide that may represent a potential alternative to antibiotics.

Inducible expression of defensins and cathelicidins by nutrients and associated regulatory mechanisms

Host defense peptides (HDPs) are crucial components of the body's first line of defense that protect organisms from infections and mediate immune responses. Defensins and cathelicidins are the two most important families of HDPs in mammals. In this review, we summarize the nutrients that are involved in inducible expression of endogenous defensins and cathelicidins. In addition, the mitogen-activated protein kinases (MAPK), nuclear factor kappa B (NF-κB) and histone deacetylase (HDAC) signaling pathways that play vital roles in the induction of defensin and cathelicidin expression are highlighted. Endogenous defensins and cathelicidins induced by nutrients may be potential alternatives to antibiotic treatments against infection and diseases. This review mainly focuses on the inducible expression and regulatory mechanisms of defensins and cathelicidins in multiple species by different nutrients and the potential applications of defensin- and cathelicidin-inducing nutrients.

Involvement of a short-type peptidoglycan recognition protein (PGRP) from Chinese giant salamanders Andrias davidianus in the immune response against bacterial infection

PGRPs (Peptidoglycan recognition proteins) could recognize peptidoglycan and play vital roles in innate immunity among different animals. Till present, the functions of PGRP have been studied in various animals, but few reports have studied the amphibian PGRPs. In the current research, a short type PGRP was identified from Chinese giant salamander and its involvement in the innate immunity was studied. The ORF of AdPGRP-SC2 cDNA was 573 bp, which encoded 190 amino acids, and contained a PGRP and an amidase_2 domain. The qPCR analysis revealed that AdPGRP-SC2 mRNA transcripts expressed in different tissues, with the highest expression level in muscle, intestine and spleen. Results of immune challenges with peptidoglycan (PGN) demonstrated that expression patterns of AdPGRP-SC2 were significantly up-regulated in erythrocyte and spleen at the early injection stage. The recombinant AdPGRP-SC2 protein was successfully produced and purified, and it could show binding affinity to different bacteria. In the presence of Zn²⁺, the rAdPGRP-SC2 could exhibit a broad PAMPs binding activities, strongly agglutinate bacteria and exhibit amidase enzyme activity. Collectively, these data indicate AdPGRP-SC2 could act as PRR to recognize the invading microorganisms and as the antimicrobial effectors during the innate immune response of A. davidianus.

Characterization and function of the T-box 1 gene in Chinese giant salamander Andrias davidianus

We characterized the Andrias davidianus T-box 1 (Tbx1) gene. Tbx1 expression was high in testis and low in other examined tissues. Immunohistochemistry detected tbx1 expression in somatic and germ cells 62 days post-hatching (dph), prior to gonad differentiation. At 210 dph, after gonad differentiation, tbx1 was expressed in spermatogonia and testis somatic cells and in granulosa cells in ovary. Tbx1 expression was up-regulated in ovary after high temperature treatment. In the neomale, tbx1 expression showed a similar profile to normal males, and vice-versa for genetic male. Over-expression of tbx1 in females after injection of TBX1 protein down-regulated the female-biased genes cyp19a and foxl2 and up-regulated the male-biased amh gene. When tbx1 was knocked down by tbx1/siRNA, cyp19a and foxl2 expression was up-regulated, and expression of amh, cyp26a, dmrt1, and wt1 was down-regulated. Results suggest that tbx1 influenced sex-related gene expression and participates in regulation of A. davidianus testis development.

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.

Mining the tree of life: Host defense peptides as antiviral therapeutics

Discovering new therapeutics for human viral diseases is important for combatting emerging infectious viruses and omnipresent circulating viruses as well as those that can become resistant to the drugs we currently have available. The innate host defense peptide (HDP) repertoire present in animals is a wealth of potential antimicrobial agents that could be mined to meet these needs. While much of the body of research regarding HDPs is in the context of bacteria, there is increasing evidence that they can be an effective source for antivirals. Peptides can be identified in a number of ways, including eco-conservation-minded approaches. Those shown to have antiviral properties can be modified to exhibit desired properties as the relationship between structure and function is elucidated and then developed into therapeutics for human use. This review looks at the discovery and therapeutic potential of HDPs for human viral infections.