A frog cathelicidin peptide effectively promotes cutaneous wound healing in mice

Assessment of the antimicrobial and immunomodulatory activity of QS-CATH, a promising therapeutic agent isolated from the Chinese spiny frogs (Quasipaa spinosa)

Cathelicidins are important antimicrobial peptides in various vertebrate species where they are crucial parts of the innate immune system. The current understanding of amphibian cathelicidins is limited, particularly with regard to their immunomodulatory effects. To address this knowledge gap, we produced the cDNA sequence of the cathelicidin gene from a skin transcriptome of the Chinese spiny frog Quasipaa spinosa. The amino acid sequence of the Quasipaa spinosa cathelicidin (QS-CATH) was predicted to consist of a signal peptide, a cathelin domain, and a mature peptide. Comparative analysis of the QS-CATH amino acid sequence with that of other amphibian cathelicidins revealed high variability in the functional mature peptide among amphibians, whereas the cathelin domain was conserved. The QS-CATH gene was expressed in several tissues, with the highest level of expression in the spleen. Upregulation of QS-CATH after Aeromonas hydrophila infection occurred in the kidney, gut, spleen, skin, and liver. Chemically synthesized QS-CATH exhibited pronounced antibacterial activity against Shigella flexneri, Staphylococcus warneri, Escherichia coli, Salmonella enterica, and Listeria monocytogenes. Furthermore, QS-CATH disrupted the cell membrane integrity of S. flexneri, as evidenced by a lactate dehydrogenase release assay, and it hydrolyzed the genomic DNA of S. flexneri. Additionally, QS-CATH elicited chemotaxis and modulated the expression of inflammatory cytokine genes in RAW264.7 mouse leukemic monocyte/macrophage cells. These findings confirm the antimicrobial effects of amphibian cathelicidin and its ability to influence immune cell function. This will expedite the potential utilization of amphibian antimicrobial peptides as therapeutic agents.

A non-Bactericidal Cathelicidin with Antioxidant Properties Ameliorates UVB-Induced Mouse Skin Photoaging via Intracellular ROS Scavenging and Keap1/Nrf2 Pathway Activation

Cathelicidins, a category of critical host defense molecules in vertebrates, have been extensively studied for their bactericidal functions, but little is known about their non-bactericidal properties. Herein, a novel cathelicidin peptide (Atonp2) was identified from the plateau frog Nanorana ventripunctata. It did not exhibit bactericidal activity but showed significant therapeutic effects in chronic UVB radiation-induced mouse skin photoaging through inhibiting thickening, pyroptosis and inflammation in the epidermis, while inhibiting cellular senescence, collagen fibre breakage and type Ⅰ collagen reduction in the dermis. Further studies indicated that Atonp2 effectively scavenged UVB-induced intracellular ROS via tyrosines at positions 9 and 10, while activating the Keap1/Nrf2 pathway to protect epidermal keratinocytes against UVB radiation, which in turn indirectly reversed the senescence and collagen degradation of dermal fibroblasts, thereby ameliorating UVB-induced skin photoaging. As such, this study identified a non-bactericidal cathelicidin peptide with potent antioxidant functions, highlighting its potential to treat and prevent skin photoaging.

Discovery of Ll-CATH: a novel cathelicidin from the Chong'an Moustache Toad (Leptobrachium liui) with antibacterial and immunomodulatory activity

BACKGROUND

Cathelicidins are vital antimicrobial peptides expressed in diverse vertebrates, crucial for immunity. Despite being a new field, amphibian cathelicidin research holds promise.

RESULTS

We isolated the cDNA sequence of the cathelicidin (Ll-CATH) gene from the liver transcriptome of the Chong'an Moustache Toad (Leptobrachium liui). We confirmed the authenticity of the cDNA sequence by rapid amplification of cDNA ends and reverse transcription PCR, and obtained the Ll-CATH amino acid sequence using the Open Reading Frame Finder, an online bioinformatics tool. Its translated protein contained a cathelin domain, signal peptide, and mature peptide, confirmed by amino acid sequence. The comparative analysis showed that the mature peptides were variable between the amphibian species, while the cathelin domain was conserved. The concentration of Ll-CATH protein and the expression of its gene varied in the tissues, with the spleen showing the highest levels. The expression levels of Ll-CATH in different tissues of toads was significantly increased post infection with Aeromonas hydrophila. Chemically synthesized Ll-CATH effectively combated Proteus mirabilis, Staphylococcus epidermidis, Vibrio harveyi, V. parahaemolyticus, and V. vulnificus; disrupted the membrane of V. harveyi, hydrolyzed its DNA. Ll-CATH induced chemotaxis and modulated the expression of pro-inflammatory cytokine genes in RAW264.7 macrophages.

CONCLUSIONS

This study unveiled the antibacterial and immunomodulatory potential of amphibian cathelicidin, implying its efficacy against infections. Ll-CATH characterization expands our knowledge, emphasizing its in a bacterial infection therapy.

Promising anti-inflammatory activity of a novel designed anti-microbial peptide for wound healing

Chronic wounds can develop as a result of prolonged inflammation during the healing process, which can happen due to bacterial infection. Therefore, preventing infection and controlling inflammation can accelerate wound healing. Antimicrobial peptides have different protective properties in addition to antimicrobial activity. Some of these activities include the stimulation of cytokine or chemokine synthesis, the facilitation of chemotaxis and cell proliferation, the acceleration of cell proliferation, the induction of anti-inflammatory responses, and the promotion of wound repair. This study aimed to assess the wound healing potential of a novel in silico-designed antimicrobial peptide. Then, its anti-inflammatory activity was investigated by measuring the level of tumor necrosis factor-α (TNF-α) and transforming growth factor beta (TGF-β) as indicators of the wound healing process. In addition, the influence of the peptide on cell migration was evaluated by a scratch test on human dermal fibroblasts (HDF) and HaCaT cells as a human epidermal keratinocyte cell line. The results showed that our new peptide could act well in inhibiting TNF-α over-secretion while increasing the expression of TGF-β as an anti-inflammatory factor. This peptide showed a significant potential to stimulate HDF and HaCaT cell migration and proliferation. Therefore, using this peptide as an anti-inflammatory component of wound dressings may be promising.

Cathelicidin-HG Alleviates Sepsis-Induced Platelet Dysfunction by Inhibiting GPVI-Mediated Platelet Activation

Platelet activation contributes to sepsis development, leading to microthrombosis and increased inflammation, which results in disseminated intravascular coagulation and multiple organ dysfunction. Although Cathelicidin can alleviate sepsis, its role in sepsis regulation remains largely unexplored. In this study, we identified Cath-HG, a novel Cathelicidin from Hylarana guentheri skin, and analyzed its structure using nuclear magnetic resonance spectroscopy. The modulatory effect of Cath-HG on the symptoms of mice with sepsis induced by cecal ligation and puncture was evaluated in vivo, and the platelet count, degree of organ damage, and microthrombosis were measured. The antiplatelet aggregation activity of Cath-HG was studied in vitro, and its target was verified. Finally, we further investigated whether Cath-HG could regulate thrombosis in vivo in a FeCl3 injury-induced carotid artery model. The results showed that Cath-HG exhibited an α-helical structure in sodium dodecyl sulfate solution and effectively reduced organ inflammation and damage, improving survival in septic mice. It alleviated sepsis-induced thrombocytopenia and microthrombosis. In vitro, Cath-HG specifically inhibited collagen-induced platelet aggregation and modulated glycoprotein VI (GPVI) signaling pathways. Dot blotting, enzyme-linked immunosorbent assay, and pull-down experiments confirmed GPVI as the target of Cath-HG. Molecular docking and amino acid residue truncations/mutations identified crucial sites of Cath-HG. These findings suggest that GPVI represents a promising therapeutic target for sepsis, and Cath-HG may serve as a potential treatment for sepsis-related thrombocytopenia and thrombotic events. Additionally, identifying Cath-HG as a GPVI inhibitor provides insights for developing novel antithrombotic therapies targeting platelet activation mediated by GPVI.

A Systematic Review of the Design and Applications of Antimicrobial Peptides in Wound Healing

The sources of antimicrobial peptides (AMPs), also known as peptide-based antibiotics, are diverse, such as plants, animals, microorganisms including human leukocytes, saliva, human defense peptides, and human sweat. These natural sources provide a rich variety of AMPs with unique characteristics and potential therapeutic applications, including wound-healing and antimicrobial properties. AMPs derived from these sources have shown promise in combating a wide range of pathogens, making them valuable targets for further research and potential clinical applications. The design of AMPs for wound healing involves a meticulous process of structurally optimizing peptides to possess a unique combination of antibacterial and wound-healing characteristics. This systematic review was produced to show the design and applications of AMPs in wound healing. The terms "antimicrobial peptides AND wound healing" were used to search for articles published between September 2023 and January 2010. In the search, we found a total of 12958 articles, of which 12898 were excluded, and the remaining 60 articles were chosen for further study. This systematic review underscores the potential of AMPs as valuable tools in infection control and wound healing, showcasing their versatility and effectiveness in combating a wide range of pathogens. Overall, AMPs in wound healing display a diverse mechanism of action, influencing the inflammatory response, encouraging tissue regeneration, and aiding tissue remodeling, along with strong antibacterial activity. Furthermore, this systematic review addresses AMP toxicity studies, which include rigorous in vitro and in vivo examinations to determine potential cytotoxic effects, systemic toxicity, and any adverse responses connected with its usage in wound-healing applications.

An amphibian-derived cathelicidin accelerates cutaneous wound healing through its main regulatory effect on phagocytes

Cathelicidins are an important family of antimicrobial peptides (AMPs) involved in the innate immunity in vertebrates. The mammalian cathelicidins have been well characterized, but the relationship between structure and function in amphibian cathelicidins is still not well understood. In this study, a novel 29-residue cathelicidin antimicrobial peptide (BugaCATH) was identified from the skin of Bufo gargarizans. Unlike other AMPs, BugaCATH does not display any direct antimicrobial effects in vitro. However, it effectively promotes full-thickness wound repair in mice. Following injury, BugaCATH initiates and expedites the inflammatory stage by recruiting neutrophils and macrophages to the wound site. BugaCATH not only regulates neutrophil phagocytic activity but also stimulates the generation of cytokines (TNF-α, IL-6, and IL-1β) and chemokines (CXCL1, CXCL2, CCL2, and CCL3) in macrophages and in mice. Furthermore, it promotes macrophage M2 polarization that facilitates the conversion from a pro-inflammatory macrophage-dominated wound environment to an anti-inflammatory one during the mid to late stages, which is crucial for reducing inflammation and effective wound repair. The MAPK (ERK, JNK, and p38) and NF-κB-NLRP3 signaling pathways are involved in the activity. Moreover, BugaCATH directly enhances the migration of keratinocytes and vascular endothelial cells without affecting their proliferation. Notably, BugaCATH significantly improves the proliferation of keratinocytes and endothelial cells in the presence of macrophages. The current study revealed that in addition to proliferation of keratinocytes and endothelial cells, BugaCATH possesses the ability to modulate inflammatory processes during skin injury through its regulatory effect on phagocytes. The combination of these capabilities makes BugaCATH a potent candidate for skin wound therapy.

Recent Advancement in Novel Wound Healing Therapies by Using Antimicrobial Peptides Derived from Humans and Amphibians

The skin is the biggest organ in the human body. It is the first line of protection against invading pathogens and the starting point for the immune system. The focus of this review is on the use of amphibian-derived peptides and antimicrobial peptides (AMPs) in the treatment of wound healing. When skin is injured, a chain reaction begins that includes inflammation, the formation of new tissue, and remodelling of existing tissue to aid in the healing process. Collaborating with non-immune cells, resident and recruited immune cells in the skin remove foreign invaders and debris, then direct the repair and regeneration of injured host tissues. Restoration of normal structure and function requires the healing of damaged tissues. However, a major issue that slows wound healing is infection. AMPs are just one type of host-defense chemicals that have developed in multicellular animals to regulate the immune response and limit microbial proliferation in response to various types of biological or physical stress. Therefore, peptides isolated from amphibians represent novel therapeutic tools and approaches for regenerating damaged skin. Peptides that speed up the healing process could be used as therapeutic lead molecules in future research into novel drugs. AMPs and amphibian-derived peptides may be endogenous mediators of wound healing and treat non-life-threatening skin and epithelial lesions. Thus, the present article was drafted with to incorporate different peptides used in wound healing, their method of preparation and routes of administration.

Revitalizing Skin Repair: Unveiling the Healing Power of Livisin, a Natural Peptide Calcium Mimetic

When the skin is damaged, accelerating the repair of skin trauma and promoting the recovery of tissue function are crucial considerations in clinical treatment. Previously, we isolated and identified an active peptide (livisin) from the skin secretion of the frog Odorrana livida. Livisin exhibited strong protease inhibitory activity, water solubility, and stability, yet its wound-healing properties have not yet been studied. In this study, we assessed the impact of livisin on wound healing and investigated the underlying mechanism contributing to its effect. Our findings revealed livisin effectively stimulated the migration of keratinocytes, with the underlying mechanisms involved the activation of CaSR as a peptide calcium mimetic. This activation resulted in the stimulation of the CaSR/E-cadherin/EGFR/ERK signaling pathways. Moreover, the therapeutic effects of livisin were partially reduced by blocking the CaSR/E-cadherin/EGFR/ERK signaling pathway. The interaction between livisin and CaSR was further investigated by molecular docking. Additionally, studies using a mouse full-thickness wound model demonstrated livisin could accelerate skin wound healing by promoting re-epithelialization and collagen deposition. In conclusion, our study provides experimental evidence supporting the use of livisin in skin wound healing, highlighting its potential as an effective therapeutic option.

Peptide TK-HR from the Skin of Chinese Folk Medicine Frog Hoplobatrachus Rugulosus Accelerates Wound Healing via the Activation of the Neurokinin-1 Receptor

Wound healing is a complex process and remains a considerable challenge in clinical trials due to the lack of ideal therapeutic drugs. Here, a new peptide TK-HR identified from the skin of the frog Hoplobatrachus rugulosus was tested for its ability to heal cutaneous wounds in mice. Topical application of TK-HR at doses of 50-200 μg/mL significantly accelerated wound closure without causing any adverse effects in the animals. In vitro and in vivo investigations proved the regulatory role of the peptide on neutrophils, macrophages, keratinocytes, and vein endothelial cells involved in the inflammatory, proliferative, and remodeling phases of wound healing. Notably, TK-HR activated the MAPK and TGF-β-Smad signaling pathways by acting on NK1R in RAW264.7 cells and mice. The current work has identified that TK-HR is a potent wound healing regulator that can be applied for the treatment of wounds, including diabetic foot ulcers and infected wounds, in the future.

Evaluation of an antibacterial peptide-loaded amniotic membrane/silk fibroin electrospun nanofiber in wound healing

Antimicrobial peptides (AMPs) are among the compounds that have significant potential to deal with infectious skin wounds. Using wound dressings or skin scaffolds containing AMPs can be an effective way to overcome infections caused by antibiotic-resistant strains. In this study, we developed an amniotic membrane-based skin scaffold using silk fibroin to improve mechanical properties and CM11 peptide as an antimicrobial peptide. The peptide was coated on the scaffold using the soaking method. The fabricated scaffold was characterised by SEM and FTIR, and their mechanical strength, biodegradation, peptide release, and cell cytotoxicity analyses were performed. Then, their antimicrobial activity was measured against antibiotic-resistant strains of Pseudomonas aeruginosa and Staphylococcus aureus. The in vivo biocompatibility of this scaffold was evaluated by subcutaneously implanting it under the skin of the mouse and counting lymphocytes and macrophages in the implanted area. Finally, the regenerative ability of the scaffold was analyzed in the mouse full-thickness wound model by measuring the wound diameter, H&E staining, and examining the expression rate of genes involved in the wound healing process. The developed scaffolds exerted an inhibiting effect on the bacteria growth, indicating their proper antimicrobial property. In vivo biocompatibility results showed no significant count of macrophages and lymphocytes between the test and control groups. The wound closure rate was significantly higher in the wound covered with fibroin electrospun-amniotic membrane loaded with 32 μg/mL CM11, where the relative expression rates of collagen I, collagen III, TGF-β1 and TGF-β3 were higher compared with the other groups.

Efficacy of LL-37 cream in enhancing healing of diabetic foot ulcer: a randomized double-blind controlled trial

Wound healing in DFU (diabetic foot ulcer) has prolonged inflammation phase and defective granulation tissue formation. LL-37 has antimicrobial property, induces angiogenesis, and keratinocyte migration and proliferation. This study analyzes the efficacy of LL-37 cream in enhancing wound healing rate and decreasing the levels of IL-1α, TNF-α, and the number of aerobic bacteria colonization in DFU with mild infection. This study was conducted from January 2020 to June 2021 in Jakarta. Subjects were instructed to apply either LL-37 cream or placebo cream twice a week for 4 weeks. Wounds were measured on days 7, 14, 21, and 28 and processed with ImageJ. The levels of LL-37, IL-1α, and TNF-α from wound fluid were measured using ELISA. The number of aerobic bacteria colonization was counted from the isolate grown in culture. The levels of LL-37 in DFU at baseline were equally low in both groups which were 1.07 (0.37-4.96) ng/mg protein in the LL-37 group and 1.11 (0.24-2.09) ng/mg protein in the placebo group. The increase in granulation index was consistently greater in the LL-37 group on days 7, 14, 21, and 28 (p = 0.031, 0.009, 0.006, and 0.037, respectively). The levels of IL-1α and TNF-α increased in both groups on days 14 and 21 (p > 0.05). The decrease in the number of aerobic bacteria colonization was greater in the LL-37 group on days 7, 14 and 21, but greater in the placebo group on day 28 (p > 0.05). In conclusion, LL-37 cream enhanced the healing rate of DFU with mild infection, but did not decrease the levels of IL-1α and TNF-α and the number of aerobic bacteria colonization. This trial is registered at ClinicalTrials.gov, number NCT04098562.

Cath-DM-NT, a peptide derived from the skin of Duttaphrynus melanostictus, shows dual lectin-like and antioxidant activity

Chansu, a mixture extracted from Duttaphrynus melanostictus or Bufo gargarizans Cantor, is a traditional Chinese medicine with a broad range of medical applications. However, the peptides/proteins in it have not received adequate attention. Herein, a Cathelicidin-DM-derived peptide named Cath-DM-NT was identified from the skin of D. melanostictus. Previous studies have shown that Cathelicidin-DM has significant antibacterial activity, while Cath-DM-NT has no antibacterial activity. In this study, Cath-DM-NT is found to have lectin-like activity which can agglutinate erythrocytes and bacteria, and bind to lipopolysaccharide (LPS). In addition, Cath-DM-NT has antioxidant activity, which can scavenge 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH), and nitric oxide (NO) radicals and reduce Fe3+. Consistently, Cath-DM-NT can protect PC12 cells from H2O2-induced oxidative damage and carrageenan-induced paw edema, reduce malondialdehyde (MDA) and reactive oxygen species (ROS) accumulation, and restore superoxide dismutase (SOD) and glutathione (GSH) levels. Our study suggests that Cath-DM-NT can serve as a lead compound for the development of drugs with dual lectin and antioxidant effects.

Enhancing Wound Healing: A Novel Topical Emulsion Combining CW49 Peptide and Lavender Essential Oil for Accelerated Regeneration and Antibacterial Protection

Wound healing is a complex process involving blood cells, extracellular matrix, and parenchymal cells. Research on biomimetics in amphibian skin has identified the CW49 peptide from Odorrana grahami, which has been demonstrated to promote wound regeneration. Additionally, lavender essential oil exhibits anti-inflammatory and antibacterial activities. Given these considerations, we propose an innovative emulsion that combines the CW49 peptide with lavender oil. This novel formulation could serve as a potent topical treatment, potentially fostering the regeneration of damaged tissues and providing robust antibacterial protection for skin wounds. This study investigates the physicochemical properties, biocompatibility, and in vitro regenerative capacity of the active components and the emulsion. The results show that the emulsion possesses appropriate rheological characteristics for topical application. Both the CW49 peptide and lavender oil exhibit high viability in human keratinocytes, indicating their biocompatibility. The emulsion induces hemolysis and platelet aggregation, an expected behavior for such topical treatments. Furthermore, the lavender-oil emulsion demonstrates antibacterial activity against both Gram-positive and Gram-negative bacterial strains. Finally, the regenerative potential of the emulsion and its active components is confirmed in a 2D wound model using human keratinocytes. In conclusion, the formulated emulsion, which combines the CW49 peptide and lavender oil, shows great promise as a topical treatment for wound healing. Further research is needed to validate these findings in more advanced in vitro models and in vivo settings, potentially leading to improved wound-care management and novel therapeutic options for patients with skin injuries.

Characterization and mechanism of action of amphibian-derived wound-healing-promoting peptides

Wound-healing-promoting peptides are excellent candidates for developing wound-healing agents due to their small size and low production cost. Amphibians are one of the major sources of bioactive peptides, including wound-healing-promoting peptides. So far, a series of wound-healing-promoting peptides have been characterized from amphibians. We hereby summarized the amphibian-derived wound-healing-promoting peptides and their mechanism of action. Among these peptides, two peptides (tylotoin and TK-CATH) were characterized from salamanders, and twenty five peptides were characterized from frogs. These peptides generally have small sizes with 5-80 amino acid residues, nine peptides (tiger17, cathelicidin-NV, cathelicidin-DM, OM-LV20, brevinin-2Ta, brevinin-2PN, tylotoin, Bv8-AJ, and RL-QN15) have intramolecular disulfide bonds, seven peptides (temporin A, temporin B, esculentin-1a, tiger17, Pse-T2, DMS-PS2, FW-1, and FW-2) are amidated at the C-terminus, and the others are linear peptides without modifications. They all efficiently accelerated the healing of skin wounds or photodamage in mice or rats. They selectively promoted the proliferation and migration of keratinocytes and fibroblasts, recruited neutrophils and macrophages to wounds, and regulated the immune response of neutrophils and macrophages in wounds, which were essential for wound healing. Interestingly, MSI-1, Pse-T2, cathelicidin-DM, brevinin-2Ta, brevinin-2PN, and DMS-PS2 were just antimicrobial peptides, but they also significantly promoted the healing of infected wounds by clearing off bacteria. Considering the small size, high efficiency, and definite mechanism, amphibian-derived wound-healing-promoting peptides might be excellent candidates for developing novel wound-healing-promoting agents in future.

Amphibian-derived wound healing peptides: chemical molecular treasure trove for skin wound treatment

Amphibian-derived wound healing peptides thus offer new intervention measures and strategies for skin wound tissue regeneration. As novel drug lead molecules, wound healing peptides can help analyze new mechanisms and discover new drug targets. Previous studies have identified various novel wound healing peptides and analyzed novel mechanisms in wound healing, especially competing endogenous RNAs (ceRNAs) (e.g., inhibition of miR-663a promotes skin repair). In this paper, we review amphibian-derived wound healing peptides, including the acquisition, identification, and activity of peptides, a combination of peptides with other materials, and the analysis of underlying mechanisms, to better understand the characteristics of wound healing peptides and to provide a molecular template for the development of new wound repair drugs.

Disease state associated with chronic toe lesions in hellbenders may alter anti-chytrid skin defenses

Hellbenders (Cryptobranchus alleganiensis) are large, aquatic salamanders from the eastern United States. Both subspecies, eastern and Ozark hellbenders, have experienced declines resulting in federal listing of Ozark hellbenders. The globally distributed chytrid fungus, Batrachochytrium dendrobatidis (Bd) has been detected in both subspecies, and Batrachochytrium salamandrivorans (Bsal) poses a new threat if introduced into North America. Ozark hellbenders also suffer a high prevalence of toe lesions of unknown etiology, with changes in host immunocompetence hypothesized to contribute. Antimicrobial peptides (AMPs) secreted from dermal granular glands may play a role in hellbender health. We collected skin secretions from free-ranging hellbenders and enriched them for small cationic peptides used for growth inhibition assays against Bd and Bsal. Generalized linear mixed models revealed the presence of active toe lesions as the strongest and only significant predictor of decreased Bd inhibition by skin peptides. We also found skin secretions were more inhibitory of Bsal than Bd. MALDI-TOF mass spectrometry revealed candidate peptides responsible for anti-chytrid activity. Results support the hypothesis that hellbender skin secretions are important for innate immunity against chytrid pathogens, and decreased production or release of skin peptides may be linked to other sub-lethal effects of disease associated with toe lesions.

Bioactive compounds: a goldmine for defining new strategies against pathogenic bacterial biofilms?

Most human infectious diseases are caused by microorganisms growing as biofilms. These three-dimensional self-organized communities are embedded in a dense matrix allowing microorganisms to persistently inhabit abiotic and biotic surfaces due to increased resistance to both antibiotics and effectors of the immune system. Consequently, there is an urgent need for novel strategies to control biofilm-associated infections. Natural products offer a vast array of chemical structures and possess a wide variety of biological properties; therefore, they have been and continue to be exploited in the search for potential biofilm inhibitors with a specific or multi-locus mechanism of action. This review provides an updated discussion of the major bioactive compounds isolated from several natural sources - such as plants, lichens, algae, microorganisms, animals, and humans - with the potential to inhibit biofilm formation and/or to disperse established biofilms by bacterial pathogens. Despite the very large number of bioactive products, their exact mechanism of action often remains to be clarified and, in some cases, the identity of the active molecule is still unknown. This knowledge gap should be filled thus allowing development of these products not only as novel drugs to combat bacterial biofilms, but also as antibiotic adjuvants to restore the therapeutic efficacy of current antibiotics.

Review on Extraction, Modification, and Synthesis of Natural Peptides and Their Beneficial Effects on Skin

Peptides, functional nutrients with a size between those of large proteins and small amino acids, are easily absorbed by the human body. Therefore, they are seeing increasing use in clinical medicine and have revealed immunomodulatory and anti-inflammatory properties which could make them effective in healing skin wounds. This review sorted and summarized the relevant literature about peptides during the past decade. Recent works on the extraction, modification and synthesis of peptides were reviewed. Importantly, the unique beneficial effects of peptides on the skin were extensively explored, providing ideas for the development and innovation of peptides and laying a knowledge foundation for the clinical application of peptides.

Characteristics of antimicrobial peptide OaBac5mini and its bactericidal mechanism against Escherichia coli

Introduction

Antimicrobial peptides (AMPs) play an important role in defending against the attack of pathogenic microorganisms. Among them, the proline-rich antibacterial peptides (PrAMPs) have been attracting close attention due to their simple structure, strong antibacterial activity, and low cell toxicity. OaBac5mini is an active fragment of the sheep-derived OaBac5 belonging to the PrAMPs family.

Methods

In this study, the antibacterial activity of OaBac5mini was investigated by testing the MICs against different stains of E. coli and S. aureus as well as the time-kill curve. The bactericidal mechanism was explored by determining the effect of OaBac5mini on the cell membrane. The stability and biosafety were also evaluated.

Results

The susceptibility test demonstrated that OaBac5mini showed potent antibacterial activity against the multidrug-resistant (MDR) E. coli isolates. It is noticeable that the absence of inner membrane protein SbmA in E. coli ATCC 25922 caused the MIC of OaBac5mini to increase 4-fold, implying OaBac5mini can enter into the cytoplasm via SbmA and plays its antibacterial activity. Moreover, the antibacterial activity of OaBac5mini against E. coli ATCC 25922 was not remarkably affected by the serum salts except for CaCl₂ at a physiological concentration, pH, temperature, repeated freeze-thawing and proteases (trypsin < 20 μg/mL, pepsin or proteinase K). Time-kill curve analysis showed OaBac5mini at the concentration of 200 μg/mL (8 × MICs) could effectively kill E. coli ATCC 25922 after co-incubation for 12 h. In addition, OaBac5mini was not hemolytic against rabbit red blood cells and also was not cytotoxic to porcine small intestinal epithelial cells (IPEC-J2). Bioinformatic analysis indicated that OaBac5mini is a linear peptide with 8 net positive charges. Furthermore, OaBac5mini significantly increased the outer membrane permeability and impaired the inner membrane integrity and ultrastructure of E. coli ATCC25922.

Conclusion

OaBac5mini is a stable and potent PrAMP that kills E. coli by two different modes of action - inhibiting intracellular target(s) and damaging cell membrane.

A Frog-Derived Cathelicidin Peptide with Dual Antimicrobial and Immunomodulatory Activities Effectively Ameliorates Staphylococcus aureus-Induced Peritonitis in Mice

As antimicrobial resistance poses an increasing threat to public health, it is urgent to develop new antimicrobial agents. In this paper, we identify a novel 30-residue peptide (Nv-CATH, NCNFLCKVKQRLRSVSSTSHIGMAIPRPRG) from the skin of the frog Nanorana ventripunctata, which belongs to the cathelicidin family. Nv-CATH exhibited broad-spectrum antimicrobial activity against Gram-positive and Gram-negative bacteria. Nv-CATH significantly protected mice from lethal infections caused by Staphylococcus aureus. Furthermore, the peptide suppressed excessive and harmful inflammatory responses by repressing the production of NO, IL-6, TNF-α, and IL-1β. The NF-κB-NLRP3 and MAPK inflammatory signaling pathways were involved in the protection in vitro and in vivo. Nv-CATH also modulated macrophage/monocyte and neutrophil trafficking to the infection site by stimulating CXCL1, CXCL2, and CCL2 production in macrophages. Nv-CATH augmented immunocyte-mediated bacterial killing by modestly promoting neutrophils' phagocytosis and PMA-induced NET formation. Thus, Nv-CATH protects mice against bacterial infection by antimicrobial-immunomodulatory duality. The combination of these two characteristics makes Nv-CATH a promising molecule template for the development of novel antimicrobial and antibiotic-resistant agents.

Brevinin-2PN, an antimicrobial peptide identified from dark-spotted frog (Pelophylax nigromaculatus), exhibits wound-healing activity

Brevinins exhibit a wide range of structural features and strong biological activities. Brevinin-2, derived from several amphibians, has shown antimicrobial activities. However, little is known about the wound-healing activity of brevinin-2. In this study, brevinin-2 cDNA was identified from the skin transcriptome of the dark-spotted frog (Pelophylax nigromaculatus) and it comprises a signal peptide, a propeptide, and a mature peptide. Sequence alignment with brevinin-2 derived from other amphibians showed variability of the mature peptide, and the presence of a C-terminal cyclic heptapeptide domain (Cys-Lys-Xaa4-Cys) in the mature peptide. Dark-spotted frog brevinin-2 belonged to the brevinin-2 cluster and was closely related to brevinin-2HB1 from Pelophylax hubeiensis. Synthetic dark-spotted frog brevinin-2 mature peptide (brevinin-2PN) exhibited antibacterial activity against several pathogens by destroying cell membrane integrity and hydrolysis of genomic DNA. Brevinin-2PN exhibited significant wound-healing activity by accelerating the healing of human skin fibroblast cell scratches, influencing cell migration, and stimulating gene expression of growth factors.

Wound healing mechanism of antimicrobial peptide cathelicidin-DM

Background and Purpose: Chronic wound infections and the development of antibiotic resistance are serious clinical problems that affect millions of people worldwide. Cathelicidin-DM, an antimicrobial peptide from Duttaphrynus melanostictus, has powerful antimicrobial activity and wound healing efficacy. So, it could be a potential candidate to address this problem. In this paper, we investigate the wound healing mechanism of cathelicidin-DM to establish a basis for preclinical studies of the drug. Experimental Approach: The effects of cathelicidin-DM on cell proliferation and migration, cytokines, and mitogen-activated protein kinase (MAPK) signaling pathways were examined. Then mice whole skin wound model was constructed to evaluate the wound healing activity of cathelicidin-DM, and further histological changes in the wounds were assessed by hematoxylin-eosin staining (H&E) and immunohistochemical assays. Key Results: Cathelicidin-DM promotes the proliferation of HaCaT, HSF, and HUVEC cells in a concentration-dependent manner and the migration of HSF, HUVEC, and RAW.264.7 cells. Moreover,cathelicidin-DM can involve in wound healing through activation of the MAPK signaling pathway by upregulating phosphorylation of ERK, JNK, and P38. However, cathelicidin-DM didn't affect the secretion of IL-6 and TNF-α. At the animal level, cathelicidin-DM accelerated skin wound healing and early debridement in mice as well as promoted re-epithelialization and granulation tissue formation, α-SMA expression, and collagen I deposition in mice. Conclusion and Implications: Our data suggest that cathelicidin-DM can be engaged in the healing of infected and non-infected wounds through multiple pathways, providing a new strategy for the treatment of infected chronic wounds.

A Novel Peptide from Polypedates megacephalus Promotes Wound Healing in Mice

Amphibian skin contains wound-healing peptides, antimicrobial peptides, and insulin-releasing peptides, which give their skin a strong regeneration ability to adapt to a complex and harsh living environment. In the current research, a novel wound-healing promoting peptide, PM-7, was identified from the skin secretions of Polypedates megacephalus, which has an amino acid sequence of FLNWRRILFLKVVR and shares no structural similarity with any peptides described before. It displays the activity of promoting wound healing in mice. Moreover, PM-7 exhibits the function of enhancing proliferation and migration in HUVEC and HSF cells by affecting the MAPK signaling pathway. Considering its favorable traits as a novel peptide that significantly promotes wound healing, PM-7 can be a potential candidate in the development of novel wound-repairing drugs.

Lactococcus lactis engineered to deliver hCAP18 cDNA alleviates DNBS-induced colitis in C57BL/6 mice by promoting IL17A and IL10 cytokine expression

With its antimicrobial and immunomodulating properties, the cathelicidin (LL37) plays an important role in innate immune system. Here, we attempted to alleviate chemically induced colitis using a lactococci strain that either directly expressed the precursor to LL37, hCAP18 (LL-pSEC:hCAP18), or delivered hCAP18 cDNA to host cells under the control of the cytomegalovirus promoter (LL-Probi-H1:hCAP18). We also investigated whether the alleviation of symptoms could be explained through modification of the gut microbiota by hCAP18. Mice were administered daily doses of LL-pSEC:hCAP18 or LL-Probi-H1:hCAP18. On day 7, colitis was induced by DNBS. During autopsy, we assessed macroscopic tissue damage in the colon and collected tissue samples for the characterization of inflammation markers and histological analysis. Feces were collected at day 7 for 16S DNA sequencing. We also performed a fecal transplant experiment in which mice underwent colon washing and received feces from Lactococcus lactis-treated mice before DNBS-colitis induction. Treatment with LL-Probi-H1:hCAP18 reduced the severity of colitis symptoms. The protective effects were accompanied by increased levels of IL17A and IL10 in mesenteric lymph node cells. L. lactis administration altered the abundance of Lachnospiraceae and Muribaculaceae. However, fecal transplant from L. lactis-treated mice did not improve DNBS-induced symptoms in recipient mice.

Cathelicidin-NV from Nanorana ventripunctata effectively protects HaCaT cells, ameliorating ultraviolet B-induced skin photoaging

Cathelicidins are diverse effector molecules in the vertebrate immune system and are related to immune regulation, inflammatory response, wound healing, and blood vessel formation. However, little is known about their free radical scavenging ability, especially in vivo. In this study, a cathelicidin molecule (cathelicidin-NV, ARGKKECKDDRCRLLMKRGSFSYV) previously identified from the spot-bellied plateau frog (Nanorana ventripunctata) (Anura, Dicroglossidae, Dicroglossinae) by us was shown to alleviate ultraviolet B (UVB)-induced skin photoaging in mice. Cathelicidin-NV effectively suppressed cytotoxicity, DNA fragmentation, apoptosis and reduced the protein expression levels of JNK, c-Jun, and MMP-1, which are involved in the regulation of collagen degradation in HaCaT cells induced by UVB irradiation. Furthermore, cathelicidin-NV also scavenged UVB-induced intracellular reactive oxygen species (ROS). Taken together, cathelicidin-NV directly scavenged excessive intracellular ROS to protect HaCaT cells, and subsequently alleviated UVB-induced skin photoaging. This study extends reports on the antioxidant function of the cathelicidin family. In addition, the properties of cathelicidin-NV make it an excellent candidate for the prevention and treatment of UV-induced skin photoaging.

Gill and Liver Transcript Expression Changes Associated With Gill Damage in Atlantic Salmon (Salmo salar)

Gill damage represents a significant challenge in the teleost fish aquaculture industry globally, due to the gill's involvement in several vital functions and direct contact with the surrounding environment. To examine the local and systemic effects accompanying gill damage (which is likely to negatively affect gill function) of Atlantic salmon, we performed a field sampling to collect gill and liver tissue after several environmental insults (e.g., harmful algal blooms). Before sampling, gills were visually inspected and gill damage was scored; gill scores were assigned from pristine [gill score 0 (GS0)] to severely damaged gills (GS3). Using a 44K salmonid microarray platform, we aimed to compare the transcriptomes of pristine and moderately damaged (i.e., GS2) gill tissue. Rank Products analysis (5% percentage of false-positives) identified 254 and 34 upregulated and downregulated probes, respectively, in GS2 compared with GS0. Differentially expressed probes represented genes associated with functions including gill remodeling, wound healing, and stress and immune responses. We performed gill and liver qPCR for all four gill damage scores using microarray-identified and other damage-associated biomarker genes. Transcripts related to wound healing (e.g., neb and klhl41b) were significantly upregulated in GS2 compared with GS0 in the gills. Also, transcripts associated with immune and stress-relevant pathways were dysregulated (e.g., downregulation of snaclec 1-like and upregulation of igkv3) in GS2 compared with GS0 gills. The livers of salmon with moderate gill damage (i.e., GS2) showed significant upregulation of transcripts related to wound healing (i.e., chtop), apoptosis (e.g., bnip3l), blood coagulation (e.g., f2 and serpind1b), transcription regulation (i.e., pparg), and stress-responses (e.g., cyp3a27) compared with livers of GS0 fish. We performed principal component analysis (PCA) using transcript levels for gill and liver separately. The gill PCA showed that PC1 significantly separated GS2 from all other gill scores. The genes contributing most to this separation were pgam2, des, neb, tnnt2, and myom1. The liver PCA showed that PC1 significantly separated GS2 from GS0; levels of hsp70, cyp3a27, pparg, chtop, and serpind1b were the highest contributors to this separation. Also, hepatic acute phase biomarkers (e.g., serpind1b and f2) were positively correlated to each other and to gill damage. Gill damage-responsive biomarker genes and associated qPCR assays arising from this study will be valuable in future research aimed at developing therapeutic diets to improve farmed salmon welfare.

A Frog Peptide Ameliorates Skin Photoaging Through Scavenging Reactive Oxygen Species

Although many bioactive peptides have been identified from the frog skins, their protective effects and the molecular mechanisms against skin photodamage are still poorly understood. In this study, a novel 20-residue peptide (antioxidin-NV, GWANTLKNVAGGLCKMTGAA) was characterized from the skin of plateau frog Nanorana ventripunctata. Antioxidin-NV obviously decreased skin erythema, thickness and wrinkle formation induced by Ultraviolet (UV) B exposure in hairless mice. In UVB-irradiated keratinocytes (HaCaT cells) and hairless mice, it effectively inhibited DNA damage through reducing p-Histone H2A.X (γH2AX) expression, alleviated cell apoptosis by decreasing the expression of apoptosis-specific protein (cleaved caspase 3), and reduced interleukin-6 (IL-6) production via blocking UVB-activated Toll-like receptor 4 (TLR4)/p38/JNK/NF-κB signaling. In UVB-irradiated human skin fibroblasts (HSF cells) and hairless mice, it effectively restored HSF cells survival rate, and rescued α-SMA accumulation and collagen (especially type I collagen) production by restoring transforming growth factor-β1 (TGF-β1)/Smad2 signaling. We found that antioxidin-NV directly and rapidly scavenged intracellular and mitochondrial ROS in HaCaT cells upon UVB irradiation, and quickly eliminated the artificial free radicals, 2, 2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS⁺). Taken together, antioxidin-NV directly and rapidly scavenged excessive ROS upon UVB irradiation, subsequently alleviated UVB-induced DNA damage, cell apoptosis, and inflammatory response, thus protecting against UVB-induced skin photoaging. These properties makes antioxidin-NV an excellent candidate for the development of novel anti-photoaging agent.

Mesoporous polydopamine nanoparticles carrying peptide RL-QN15 show potential for skin wound therapy

BACKGROUND

Skin wound healing remains a considerable clinical challenge, thus stressing the urgent need for the development of new interventions to promote repair. Recent researches indicate that both peptides and nanoparticles may be potential therapies for the treatment of skin wounds.

METHODS

In the current study, the mesoporous polydopamine (MPDA) nanoparticles were prepared and the peptide RL-QN15 that was previously identified from amphibian skin secretions and exhibited significant potential as a novel prohealing agent was successfully loaded onto the MPDA nanoparticles, which was confirmed by results of analysis of scanning electron microscopy and fourier transform infrared spectroscopy. The encapsulation efficiency and sustained release rate of RL-QN15 from the nanocomposites were determined. The prohealing potency of nanocomposites were evaluated by full-thickness injured wounds in both mice and swine and burn wounds in mice.

RESULTS

Our results indicated that, compared with RL-QN15 alone, the prohealing potency of nanocomposites of MPDA and RL-QN15 in the full-thickness injured wounds and burn wounds in mice was increased by up to 50 times through the slow release of RL-QN15. Moreover, the load on the MPDA obviously increased the prohealing activities of RL-QN15 in full-thickness injured wounds in swine. In addition, the obvious increase in the prohealing potency of nanocomposites of MPDA and RL-QN15 was also proved by the results from histological analysis.

CONCLUSIONS

Based on our knowledge, this is the first research to report that the load of MPDA nanoparticles could significantly increase the prohealing potency of peptide and hence highlighted the promising potential of MPDA nanoparticles-carrying peptide RL-QN15 for skin wound therapy.

Antimicrobial peptides: mechanism of action, activity and clinical potential

The management of bacterial infections is becoming a major clinical challenge due to the rapid evolution of antibiotic resistant bacteria. As an excellent candidate to overcome antibiotic resistance, antimicrobial peptides (AMPs) that are produced from the synthetic and natural sources demonstrate a broad-spectrum antimicrobial activity with the high specificity and low toxicity. These peptides possess distinctive structures and functions by employing sophisticated mechanisms of action. This comprehensive review provides a broad overview of AMPs from the origin, structural characteristics, mechanisms of action, biological activities to clinical applications. We finally discuss the strategies to optimize and develop AMP-based treatment as the potential antimicrobial and anticancer therapeutics.

Discovery of Antioxidant Peptides from Amphibians: A Review

In recent years, bioactive peptide drugs have attracted growing attention due to the increasing difficulty in developing new drugs with novel chemical structures. In addition, many diseases are linked to excessive oxidation in the human body. Therefore, the role of peptides with antioxidant activity in counteracting diseases related to oxidative stress is worth exploring. Amphibians are a major repository for bioactive peptides that protect the skin from biotic and abiotic stresses, such as microbial infection and radiation injury. We characterized the first amphibian- derived gene-encoded antioxidant peptides in 2008. Since then, a variety of antioxidant peptides have been detected in different amphibian species. In this work, the physicochemical properties of antioxidant peptides identified from amphibians are reviewed for the first time, particularly acquisition methods, amino acid characteristics, antioxidant mechanisms, and application prospects. This review should provide a reference for advancing the identification, structural analysis, and potential therapeutic value of natural antioxidant peptides.

FM-CATH, A Novel Cathelicidin From Fejervarya Multistriata, Shows Therapeutic Potential for Treatment of CLP-Induced Sepsis

Sepsis is an exacerbated inflammatory reaction induced by severe infection. As important defensive molecules in innate immunity, several AMPs are reported to prevent septic shock. In this study, we characterized a novel cathelicidin, FM-CATH, from the frog skin of F. multistriata. FM-CATH was found to adopt an amphipathic α-helix structural in membrane-mimetic environments and possess favorable antimicrobial effects against bacteria and fungus. In addition, it triggered the agglutination of bacteria. It could also strongly bind to LPS and LTA. Additionally, FM-CATH affected the enzymatic activities of thrombin, plasmin, β-tryptase, and tPA, leading to coagulation inhibition in vitro and in vivo. Finally, we observed that FM-CATH improved survival rate and inhibited pathological alteration, bacterial count, serum biochemistry, and pro-inflammatory cytokine expression in the cecal ligation and puncture-induced sepsis mice. Taken together, these findings suggest that FM-CATH might be served as a promising agent for the treatment of sepsis.

Molecular characterization of cathelicidin in tiger frog (Hoplobatrachus rugulosus): Antimicrobial activity and immunomodulatory activity

Cathelicidins are an important antimicrobial peptide family and are expressed in many different vertebrates. They play an important role in the innate immune system of the host. However, amphibian cathelicidins are poorly understood. In this study, the cDNA of the cathelicidin gene was obtained from the skin transcriptome of tiger frog (Hoplobatrachus rugulosus). The predicted amino acid sequence of tiger frog cathelicidin (HR-CATH) comprises a signal peptide, a cathelin domain, and a mature peptide. The HR-CATH amino acid sequence alignment with other frog cathelicidins showed that the functional mature peptide is highly variable in amphibians, whereas the cathelin domain is conserved. A phylogenetic tree analysis showed that HR-CATH is most closely related to cathelicidin-NV from Nanorana ventripunctata. HR-CATH was chemically synthesized and its in vitro activity was determined. It had high antibacterial activity against Vibrio parahaemolyticus, Staphylococcus aureus, and the pathogenic bacterium Aeromonas hydrophila. HR-CATH damaged the cell membrane integrity of A. hydrophila according to a lactate dehydrogenase release assay and was able to hydrolyze the genomic DNA from A. hydrophila in a dose-dependent manner. Furthermore, in RAW264.7 cells (mouse leukemic monocyte/macrophage cell line), HR-CATH induced chemotaxis and enhanced respiratory burst. Our study shows that amphibian cathelicidin has antimicrobial activity and an immunomodulatory effect on immune cells.

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 characterization of novel bi-functional cathelicidins from the black-spotted frog (Pelophylax nigromaculata) with both anti-infective and antioxidant activities

Cathelicidins are an important family of antimicrobial peptides (AMPs), which play pivotal roles in vertebrate immune responses against microbial infections. They are regarded as potential drug leads for the development of novel antimicrobial agents and three related drugs have been developed into clinical trials. Thus, it is meaningful to identify more cathelicidins from vertebrate species. Cathelicidins from ranid frogs possess special structural characteristics and activities, but to date only 12 ranid frog cathelicidins have been identified. In the present study, two novel cathelicidins (PN-CATH1 and 2) were identified from the black-spotted frog, Pelophylax nigromaculata. PN-CATHs possess low sequence similarity with the known cathelicidins. They exhibited moderate, but broad-spectrum and rapid antimicrobial activities against the tested bacteria. They kill bacteria by mainly inducing bacterial membrane disruption and possibly generating intracellular ROS formation. They also possess potent anti-biofilm and persister cell killing activity, indicating their potential in combating infections induced by biofilms-forming bacteria. Besides direct antimicrobial activity, they exhibited potent anti-inflammatory activity by effectively inhibiting the LPS-induced production of pro-inflammatory cytokines in mouse macrophages, which could be partly ascribed to their direct LPS-neutralizing ability. Furthermore, PN-CATHs demonstrated powerful in vitro free radical scavenging activities. Ultraviolet radiation significantly increased their in vivo gene expression in frog skin. Meanwhile, they possess weak cytotoxic activity and extremely low hemolytic activity. PN-CATHs represent the first discovery of cathelicidins family AMPs with both potent anti-infective and antioxidant activities. The discovery of PN-CATHs provides potential peptide leads for the development of novel anti-infective and antioxidant drugs.

Peptides from Animal Origin: A Systematic Review on Biological Sources and Effects on Skin Wounds

Background

Skin wounds are closely correlated with opportunistic infections and sepsis risk. Due to the need of more efficient healing drugs, animal peptides are emerging as new molecular platforms to accelerate skin wound closure and to prevent and control bacterial infection.

Aim

The aim of this study was to evaluate the preclinical evidence on the impact of animal peptides on skin wound healing. In addition, we carried out a critical analysis of the studies' methodological quality. Main Methods. This systematic review was performed according to the PRISMA guidelines, using a structured search on the PubMed-Medline, Scopus, and Web of Science platforms to retrieve studies published until August 25, 2020 at 3 : 00 pm. The studies included were limited to those that used animal models, investigated the effect of animal peptides with no association with other compounds on wound healing, and that were published in English. Bias analysis and methodological quality assessments were examined through the SYRCLE's RoB tool.

Results

Thirty studies were identified using the PRISMA workflow. In general, animal peptides were effective in accelerating skin wound healing, especially by increasing cellular proliferation, neoangiogenesis, colagenogenesis, and reepithelialization. Considering standardized methodological quality indicators, we identified a marked heterogeneity in research protocols and a high risk of bias associated with limited characterization of the experimental designs.

Conclusion

Animal peptides show a remarkable healing potential with biotechnological relevance for regenerative medicine. However, rigorous experimental approaches are still required to clearly delimit the mechanisms underlying the healing effects and the risk-benefit ratio attributed to peptide-based treatments.

Cathelicidin Host Defense Peptides and Inflammatory Signaling: Striking a Balance

Host-defense peptides (HDPs) are vital components of innate immunity in all vertebrates. While their antibacterial activity toward bacterial cells was the original focus for research, their ability to modulate immune and inflammatory processes has emerged as one of their major functions in the host and as a promising approach from which to develop novel therapeutics targeting inflammation and innate immunity. In this review, with particular emphasis on the cathelicidin family of peptides, the roles of natural HDPs are examined in managing immune activation, cellular recruitment, cytokine responses, and inflammation in response to infection, as well as their contribution(s) to various inflammatory disorders and autoimmune diseases. Furthermore, we discuss current efforts to develop synthetic HDPs as therapeutics aimed at restoring balance to immune responses that are dysregulated and contribute to disease pathologies.

Cathelicidin-DM is an Antimicrobial Peptide from Duttaphrynus melanostictus and Has Wound-Healing Therapeutic Potential

Antimicrobial peptides (AMPs) are a class of templates with application potential for drug development. Amphibians are important sources of AMPs. Duttaphrynus melanostictus is the main source of traditional Chinese medicine "Chansu", which has anti-infection effect while without a clear mechanism. This study aimed to find the cathelicidin peptide in D. melanostictus and then investigate the activity in vivo and in vitro, and an AMP-encoding gene (cathelicidin-DM, GenBank: KJ820824.1) was obtained from the constructed cDNA library of D. melanostictus. The MIC test and SYTOX Green uptake were used for the evaluation of the bactericidal capacity and mechanisms. The serum stability tests were used for the evaluation of the application potential. The skin wound infection model and in vivo imaging were used for in vitro application of possibility evaluation. The results showed that cathelicidin-DM was a 37 amino acid AMP with good bactericidal ability, which was similar to melittin: both can kill bacteria within 15 min. Moreover, cathelicidin-DM exhibits good therapeutic potential in the mouse wound infection model, and it can be enriched to the site of infection for treatment. Thus, cathelicidin-DM could be a new template for antimicrobial drug development given its good antibacterial activity in vivo and in vitro.

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.

A short peptide potentially promotes the healing of skin wound

Skin wound, a common form of skin damage in daily life, remains a serious challenge in clinical treatment. Bioactive peptides with high efficiency have been considered as potential therapeutic candidates for wound healing. In this report, a novel short linear peptide, with mature peptide sequence of 'GLLSGINAEWPC' and no obvious similarity with other known bioactive peptides, was identified by genomic method from the skin of odorous frog, Odorrana andersonii Our results suggested that OA-GL12 (OA: abbreviation of species (O. andersonii), GL: two initial amino acids, 12: peptide length) obviously accelerated the scratch-healing of keratinocytes and human fibroblasts in a time- and concentration-dependent manner. Meanwhile, OA-GL12 showed significant effect in promoting the wound healing on the full-thickness skin wound model. Inflammatory assay results demonstrated that OA-GL12 induced the secretion of tumor necrosis factor (TNF) and transforming growth factor β1 (TGF-β1) on murine macrophage cell line (RAW264.7), which might explain the powerful accelerating capacity of wound healing. Moreover, results also indicated that epidermal growth factor receptor (EGFR) was involved in the mechanisms underlying the scratch-healing promoting activity of OA-GL12. In addition, OA-GL12 showed obvious free radical scavenging activity. Results supported that OA-GL12 did not exert risk in acute toxicity, hemolytic activity, and direct antibacterial activity. The remarkable effect of OA-GL12 on promoting wound healing verified in this research made it potential to be a novel template for the development of wound healing-promoting agents.

Frog Skin Innate Immune Defences: Sensing and Surviving Pathogens

Amphibian skin is a mucosal surface in direct and continuous contact with a microbially diverse and laden aquatic and/or terrestrial environment. As such, frog skin is an important innate immune organ and first line of defence against pathogens in the environment. Critical to the innate immune functions of frog skin are the maintenance of physical, chemical, cellular, and microbiological barriers and the complex network of interactions that occur across all the barriers. Despite the global decline in amphibian populations, largely as a result of emerging infectious diseases, we understand little regarding the cellular and molecular mechanisms that underlie the innate immune function of amphibian skin and defence against pathogens. In this review, we discuss the structure, cell composition and cellular junctions that contribute to the skin physical barrier, the antimicrobial peptide arsenal that, in part, comprises the chemical barrier, the pattern recognition receptors involved in recognizing pathogens and initiating innate immune responses in the skin, and the contribution of commensal microbes on the skin to pathogen defence. We briefly discuss the influence of environmental abiotic factors (natural and anthropogenic) and pathogens on the immunocompetency of frog skin defences. Although some aspects of frog innate immunity, such as antimicrobial peptides are well-studied; other components and how they contribute to the skin innate immune barrier, are lacking. Elucidating the complex network of interactions occurring at the interface of the frog's external and internal environments will yield insight into the crucial role amphibian skin plays in host defence and the environmental factors leading to compromised barrier integrity, disease, and host mortality.

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.