The Potential of Frog Skin Peptides for Anti-Infective Therapies: The Case of Esculentin-1a(1-21)NH2

Gastrointestinal digestion of food proteins: Anticancer, antihypertensive, anti-obesity, and immunomodulatory mechanisms of the derived peptides

Food proteins and their peptides play a significant role in the important biological processes and physiological functions of the body. The peptides show diverse biological benefits ranging from anticancer to antihypertensive, anti-obesity, and immunomodulatory, among others. In this review, an overview of food protein digestion in the gastrointestinal tract and the mechanisms involved was presented. As some proteins remain resistant and undigested, the multifarious factors (e.g. protein type and structure, microbial composition, pH levels and redox potential, host factors, etc.) affecting their colonic fermentation, the derived peptides, and amino acids that evade intestinal digestion are thus considered. The section that follows focuses on the mechanisms of the peptides with anticancer, antihypertensive, anti-obesity, and immunomodulatory effects. As further considerations were made, it is concluded that clinical studies targeting a clear understanding of the gastrointestinal stability, bioavailability, and safety of food-based peptides are still warranted.

An esculentin-1 homolog from a dark-spotted frog (Pelophylax nigromaculatus) possesses antibacterial and immunoregulatory properties

BACKGROUND

Esculentin-1, initially discovered in the skin secretions of pool frogs (Pelophylax lessonae), has demonstrated broad-spectrum antimicrobial activity; however, its immunomodulatory properties have received little attention.

RESULTS

In the present study, esculentin-1 cDNA was identified by analysing the skin transcriptome of the dark-spotted frog (Pelophylax nigromaculatus). Esculentin-1 from this species (esculentin-1PN) encompasses a signal peptide, an acidic spacer peptide, and a mature peptide. Sequence alignments with other amphibian esculentins-1 demonstrated conservation of the peptide, and phylogenetic tree analysis revealed its closest genetic affinity to esculentin-1P, derived from the Fukien gold-striped pond frog (Pelophylax fukienensis). Esculentin-1PN transcripts were observed in various tissues, with the skin exhibiting the highest mRNA levels. Synthetic esculentin-1PN demonstrated antibacterial activity against various pathogens, and esculentin-1PN exhibited bactericidal activity by disrupting cell membrane integrity and hydrolyzing genomic DNA. Esculentin-1PN did not stimulate chemotaxis in RAW264.7, a murine leukemic monocyte/macrophage cell line. However, it amplified the respiratory burst and augmented the pro-inflammatory cytokine gene (TNF-α and IL-1β) expression in RAW264.7 cells.

CONCLUSIONS

This novel finding highlights the immunomodulatory activity of esculentin-1PN on immune cells.

Antimicrobial peptides for novel antiviral strategies in the current post-COVID-19 pandemic

The recent pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has highlighted how urgent and necessary the discovery of new antiviral compounds is for novel therapeutic approaches. Among the various classes of molecules with antiviral activity, antimicrobial peptides (AMPs) of innate immunity are among the most promising ones, mainly due to their different mechanisms of action against viruses and additional biological properties. In this review, the main physicochemical characteristics of AMPs are described, with particular interest toward peptides derived from amphibian skin. Living in aquatic and terrestrial environments, amphibians are one of the richest sources of AMPs with different primary and secondary structures. Besides describing the various antiviral activities of these peptides and the underlying mechanism, this review aims at emphasizing the high potential of these small molecules for the development of new antiviral agents that likely reduce the selection of resistant strains.

Hylin-a1: A Pan-Inhibitor against Emerging and Re-Emerging Respiratory Viruses

Pandemic and epidemic outbreaks of respiratory viruses are a challenge for public health and social care system worldwide, leading to high mortality and morbidity among the human populations. In light of the limited efficacy of current vaccines and antiviral drugs against respiratory viral infections and the emergence and re-emergence of new viruses, novel broad-spectrum antiviral drugs are needed for the prevention and treatment of these infections. Antimicrobial peptides with an antiviral effect, also known as AVPs, have already been reported as potent inhibitors of viral infections by affecting different stages of the virus lifecycle. In the present study, we analyzed the activity of the AVP Hylin-a1, secreted by the frog Hypsiboas albopunctatus, against a wide range of respiratory viruses, including the coronaviruses HCoV-229E and SARS-CoV-2, measles virus, human parainfluenza virus type 3, and influenza virus H1N1. We report a significant inhibitory effect on infectivity in all the enveloped viruses, whereas there was a lack of activity against the naked coxsackievirus B3. Considering the enormous therapeutic potential of Hylin-a1, further experiments are required to elucidate its mechanism of action and to increase its stability by modifying the native sequence.

The Antimicrobial Peptide Esculentin-1a(1-21)NH2 Stimulates Wound Healing by Promoting Angiogenesis through the PI3K/AKT Pathway

Delayed wound healing is a persistent medical problem mainly caused by decreased angiogenesis. Esculentin-1a(1-21)NH₂ [Esc-1a(1-21)NH₂], has broad-spectrum antibacterial properties which comes from frog skins. It has shown promise as a treatment for wound healing. However, its effects on angiogenesis as well as the mechanism by which esc-1a(1-21)NH₂ enhanced wound healing remained unclear. In this study, we analyzed the structural properties and biocompatibility of esc-1a(1-21)NH₂ and evaluated its effect on wound closure using a full-thickness excision model in mice. Our results showed that esc-1a(1-21)NH₂ significantly accelerated wound healing by increasing collagen deposition and angiogenesis, characterized by elevated expression levels of platelet, endothelial cell adhesion molecule-1 (CD31) and proliferating cell nuclear antigen (PCNA). Furthermore, the angiogenic activity of esc-1a(1-21)NH₂ was confirmed in vitro by various assays. Esc-1a(1-21)NH₂ significantly promoted cell migration and cell proliferation in human umbilical vein vascular endothelial cells (HUVECs) via activation of the phosphatidylinositol 3'-kinase (PI3K)/protein kinase B (AKT) pathway, and upregulated the expression of CD31 at both mRNA and protein levels. The effect of esc-1a(1-21)NH₂ on angiogenesis was diminished by LY294002, a PI3K pathway inhibitor. Taken together, this study demonstrates that esc-1a(1-21)NH₂ accelerates wound closure in mice by promoting angiogenesis via the PI3K/AKT signaling pathway, suggesting its effective application in the treatment of wound healing.

KDEON WK-11: A short antipseudomonal peptide with promising potential

The plight of antimicrobial resistance continues to limit the availability of antibiotic treatment effective in combating resistant bacterial infections. Despite efforts made to rectify this issue and minimise its effects on both patients and the wider community, progress in this area remains minimal. Here, we de-novo designed a peptide named KDEON WK-11, building on previous work establishing effective residues and structures active in distinguished antimicrobial peptides such as lactoferrin. We assessed its antimicrobial activity against an array of bacterial strains and identified its most potent effect, against Pseudomonas aeruginosa with an MIC value of 3.12 μM, lower than its counterparts developed with similar residues and chain lengths. We then determined its anti-biofilm properties, potential mechanism of action and in vitro cytotoxicity. We identified that KDEON WK-11 had a broad range of antimicrobial activity and specific capabilities to fight Pseudomonas aeruginosa with low in vitro cytotoxicity and promising potential to express anti-lipopolysaccharide qualities, which could be exploited to expand its properties into an anti-sepsis agent.

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.

The Inhibition of DNA Viruses by the Amphibian Antimicrobial Peptide Temporin G: A Virological Study Addressing HSV-1 and JPCyV

Herpes simplex virus type-1 (HSV-1) and John Cunningham polyomavirus (JCPyV) are widely distributed DNA viruses causing mainly asymptomatic infection, but also mild to very severe diseases, especially when these viruses reach the brain. Some drugs have been developed to inhibit HSV-1 replication in host cells, but their prolonged use may induce resistance phenomena. In contrast, to date, there is no cure for JCPyV. The search for alternative drugs that can reduce viral infections without undermining the host cell is moving toward antimicrobial peptides (AMPs) of natural occurrence. These include amphibian AMPs belonging to the temporin family. Herein, we focus on temporin G (TG), showing that it strongly affects HSV-1 replication by acting either during the earliest stages of its life cycle or directly on the virion. Computational studies have revealed the ability of TG to interact with HSV-1 glycoprotein B. We also found that TG reduced JCPyV infection, probably affecting both the earliest phases of its life cycle and the viral particle, likely through an interaction with the viral capsid protein VP1. Overall, our results are promising for the development of short naturally occurring peptides as antiviral agents used to counteract diseases related to HSV-1 and JCPyV.

The Antimicrobial Peptide Esc(1-21) Synergizes with Colistin in Inhibiting the Growth and in Killing Multidrug Resistant Acinetobacter baumannii Strains

Multidrug-resistant microbial infections and the scarce availability of new antibiotics capable of eradicating them are posing a serious problem to global health security. Among the microorganisms that easily acquire resistance to antibiotics and that are the etiological cause of severe infections, there is Acinetobacter baumannii. Carbapenems are the principal agents used to treat A. baumannii infections. However, when strains develop resistance to this class of antibiotics, colistin is considered one of the last-resort drugs. However, the appearance of resistance to colistin also makes treatment of the Acinetobacter infections very difficult. Antimicrobial peptides (AMP) from the innate immunity hold promise as new alternative antibiotics due to their multiple biological properties. In this study, we characterized the activity and the membrane-perturbing mechanism of bactericidal action of a derivative of a frog-skin AMP, namely Esc(1-21), when used alone or in combination with colistin against multidrug-resistant A. baumannii clinical isolates. We found that the mixture of the two compounds had a synergistic effect in inhibiting the growth and killing of all of the tested strains. When combined at dosages below the minimal inhibitory concentration, the two drugs were also able to slow down the microbial growth and to potentiate the membrane-perturbing effect. To the best of our knowledge, this is the first report showing a synergistic effect between AMPs, i.e., Esc(1-21), and colistin against colistin-resistant A. baumannii clinical isolates, highlighting the potential clinical application of such combinational therapy.

Temporin G, an amphibian antimicrobial peptide against influenza and parainfluenza respiratory viruses: Insights into biological activity and mechanism of action

Treatment of respiratory viral infections remains a global health concern, mainly due to the inefficacy of available drugs. Therefore, the discovery of novel antiviral compounds is needed; in this context, antimicrobial peptides (AMPs) like temporins hold great promise. Here, we discovered that the harmless temporin G (TG) significantly inhibited the early life-cycle phases of influenza virus. The in vitro hemagglutinating test revealed the existence of TG interaction with the viral hemagglutinin (HA) protein. Furthermore, the hemolysis inhibition assay and the molecular docking studies confirmed a TG/HA complex formation at the level of the conserved hydrophobic stem groove of HA. Remarkably, these findings highlight the ability of TG to block the conformational rearrangements of HA2 subunit, which are essential for the viral envelope fusion with intracellular endocytic vesicles, thereby neutralizing the virus entry into the host cell. In comparison, in the case of parainfluenza virus, which penetrates host cells upon a membrane-fusion process, addition of TG to infected cells provoked ~1.2 log reduction of viral titer released in the supernatant. Nevertheless, at the same condition, an immunofluorescent assay showed that the expression of viral hemagglutinin/neuraminidase protein was not significantly reduced. This suggested a peptide-mediated block of some late steps of viral replication and therefore the impairment of the extracellular release of viral particles. Overall, our results are the first demonstration of the ability of an AMP to interfere with the replication of respiratory viruses with a different mechanism of cell entry and will open a new avenue for the development of novel therapeutic approaches against a large variety of respiratory viruses, including the recent SARS-CoV2.

D-Amino Acid-Containing Lipopeptides Derived from the Lead Peptide BP100 with Activity against Plant Pathogens

From a previous collection of lipopeptides derived from BP100, we selected 18 sequences in order to improve their biological profile. In particular, analogues containing a D-amino acid at position 4 were designed, prepared, and tested against plant pathogenic bacteria and fungi. The biological activity of these sequences was compared with that of the corresponding parent lipopeptides with all L-amino acids. In addition, the influence of the length of the hydrophobic chain on the biological activity was evaluated. Interestingly, the incorporation of a D-amino acid into lipopeptides bearing a butanoyl or a hexanoyl chain led to less hemolytic sequences and, in general, that were as active or more active than the corresponding all L-lipopeptides. The best lipopeptides were BP475 and BP485, both incorporating a D-Phe at position 4 and a butanoyl group, with MIC values between 0.8 and 6.2 µM, low hemolysis (0 and 24% at 250 µM, respectively), and low phytotoxicity. Characterization by NMR of the secondary structure of BP475 revealed that the D-Phe at position 4 disrupts the α-helix and that residues 6 to 10 are able to fold in an α-helix. This secondary structure would be responsible for the high antimicrobial activity and low hemolysis of this lipopeptide.

Antimicrobial Peptides and Proteins: From Nature's Reservoir to the Laboratory and Beyond

Rapid rise of antimicrobial resistance against conventional antimicrobials, resurgence of multidrug resistant microbes and the slowdown in the development of new classes of antimicrobials, necessitates the urgent development of alternate classes of therapeutic molecules. Antimicrobial peptides (AMPs) are small proteins present in different lifeforms in nature that provide defense against microbial infections. They have been effective components of the host defense system for a very long time. The fact that the development of resistance by the microbes against the AMPs is relatively slower or delayed compared to that against the conventional antibiotics, makes them prospective alternative therapeutics of the future. Several thousands of AMPs have been isolated from various natural sources like microorganisms, plants, insects, crustaceans, animals, humans, etc. to date. However, only a few of them have been translated commercially to the market so far. This is because of some inherent drawbacks of the naturally obtained AMPs like 1) short half-life owing to the susceptibility to protease degradation, 2) inactivity at physiological salt concentrations, 3) cytotoxicity to host cells, 4) lack of appropriate strategies for sustained and targeted delivery of the AMPs. This has led to a surge of interest in the development of synthetic AMPs which would retain or improve the antimicrobial potency along with circumventing the disadvantages of the natural analogs. The development of synthetic AMPs is inspired by natural designs and sequences and strengthened by the fusion with various synthetic elements. Generation of the synthetic designs are based on various strategies like sequence truncation, mutation, cyclization and introduction of unnatural amino acids and synthons. In this review, we have described some of the AMPs isolated from the vast repertoire of natural sources, and subsequently described the various synthetic designs that have been developed based on the templates of natural AMPs or from de novo design to make commercially viable therapeutics of the future. This review entails the journey of the AMPs from their natural sources to the laboratory.

Antimicrobial Peptides and their Multiple Effects at Sub-Inhibitory Concentrations

The frequent occurrence of multidrug-resistant strains to conventional antimicrobials has led to a clear decline in antibiotic therapies. Therefore, new molecules with different mechanisms of action are extremely necessary. Due to their unique properties, antimicrobial peptides (AMPs) represent a valid alternative to conventional antibiotics and many of them have been characterized for their activity and cytotoxicity. However, the effects that these peptides cause at concentrations below the minimum growth inhibitory concentration (MIC) have yet to be fully analyzed along with the underlying molecular mechanism. In this mini-review, the ability of AMPs to synergize with different antibiotic classes or different natural compounds is examined. Furthermore, data on microbial resistance induction are reported to highlight the importance of antibiotic resistance in the fight against infections. Finally, the effects that sub-MIC levels of AMPs can have on the bacterial pathogenicity are summarized while showing how signaling pathways can be valid therapeutic targets for the treatment of infectious diseases. All these aspects support the high potential of AMPs as lead compounds for the development of new drugs with antibacterial and immunomodulatory activities.

Natural Anti-biofilm Agents: Strategies to Control Biofilm-Forming Pathogens

Pathogenic microorganisms and their chronic pathogenicity are significant concerns in biomedical research. Biofilm-linked persistent infections are not easy to treat due to resident multidrug-resistant microbes. Low efficiency of various treatments and in vivo toxicity of available antibiotics drive the researchers toward the discovery of many effective natural anti-biofilm agents. Natural extracts and natural product-based anti-biofilm agents are more efficient than the chemically synthesized counterparts with lesser side effects. The present review primarily focuses on various natural anti-biofilm agents, i.e., phytochemicals, biosurfactants, antimicrobial peptides, and microbial enzymes along with their sources, mechanism of action via interfering in the quorum-sensing pathways, disruption of extracellular polymeric substance, adhesion mechanism, and their inhibitory concentrations existing in literature so far. This study provides a better understanding that a particular natural anti-biofilm molecule exhibits a different mode of actions and biofilm inhibitory activity against more than one pathogenic species. This information can be exploited further to improve the therapeutic strategy by a combination of more than one natural anti-biofilm compounds from diverse sources.

Naturally-Occurring Alkaloids of Plant Origin as Potential Antimicrobials against Antibiotic-Resistant Infections

Antibiotic resistance is now considered a worldwide problem that puts public health at risk. The onset of bacterial strains resistant to conventional antibiotics and the scarcity of new drugs have prompted scientific research to re-evaluate natural products as molecules with high biological and chemical potential. A class of natural compounds of significant importance is represented by alkaloids derived from higher plants. In this review, we have collected data obtained from various research groups on the antimicrobial activities of these alkaloids against conventional antibiotic-resistant strains. In addition, the structure–function relationship was described and commented on, highlighting the high potential of alkaloids as antimicrobials.

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.