Brevinin-2GHk from Sylvirana guentheri and the Design of Truncated Analogs Exhibiting the Enhancement of Antimicrobial Activity

Modification and Synergistic Studies of a Novel Frog Antimicrobial Peptide against Pseudomonas aeruginosa Biofilms

The overuse of traditional antibiotics has resulted in bacterial resistance and seriously compromised the therapeutic efficacy of traditional antibiotics, making the exploration of new antimicrobials particularly important. Several studies have shown that bioactive peptides have become an important source of new antimicrobial drugs due to their broad-spectrum antibacterial action and lack of susceptibility to resistance. In this study, a novel bioactive peptide Nigrosin-6VL was characterised from the skin secretion of the golden cross band frog, Odorrana andersonii, by using the 'shotgun' cloning strategy. Modifications on the Rana Box of Nigrosin-6VL revealed its critical role in antimicrobial functions. The peptide analogue, 2170-2R, designed to preserve the Rana Box structure while enhancing cationicity, exhibited improved therapeutic efficacy, particularly against Gram-negative bacteria, with a therapeutic value of 45.27. Synergistic studies demonstrated that 2170-2R inherits the synergistic antimicrobial activities of the parent peptides and effectively enhances the antimicrobial capacity of cefepime and gentamicin against both planktonic cells and biofilms. Specifically, 2170-2R can synergise effectively with cefepime and gentamicin against different strains of P. aeruginosa biofilms. Consequently, 2170-2R holds promise as a potent antimicrobial agent developed to combat infections induced by Pseudomonas aeruginosa.

Discovery and analysis of a novel antimicrobial peptide B1AW from the skin secretion of Amolops wuyiensis and improving the membrane-binding affinity through the construction of the lysine-introduced analogue

In the development and study of antimicrobial peptides (AMPs), researchers have kept a watchful eye on peptides from the brevinin family because of their extensive antimicrobial activities and anticancer potency. In this study, a novel brevinin peptide was isolated from the skin secretions of the Wuyi torrent frog, Amolops wuyiensis (A. wuyiensisi), named B1AW (FLPLLAGLAANFLPQIICKIARKC). B1AW displayed anti-bacterial activity against Gram-positive bacteria Staphylococcus aureus (S. aureus), methicillin-resistant Staphylococcus aureus (MRSA), and Enterococcus faecalis (E. faecalis). B1AW-K was designed to broaden the antimicrobial spectrum of B1AW. The introduction of a lysine residue generated an AMP with enhanced broad-spectrum antibacterial activity. It also displayed the ability to inhibit the growth of human prostatic cancer PC-3, non-small lung cancer H838, and glioblastoma cancer U251MG cell lines. In molecular dynamic (MD) simulations, B1AW-K had a faster approach and adsorption to the anionic membrane than B1AW. Therefore, B1AW-K was considered a drug prototype with a dual effect, which deserves further clinical investigation and validation.

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.

Targeted Modification and Structure-Activity Study of GL-29, an Analogue of the Antimicrobial Peptide Palustrin-2ISb

Antimicrobial peptides (AMPs) are considered as promising antimicrobial agents due to their potent bioactivity. Palustrin-2 peptides were previously found to exhibit broad-spectrum antimicrobial activity with low haemolytic activity. Therefore, GL-29 was used as a template for further modification and study. Firstly, the truncated analogue, GL-22, was designed to examine the function of the ‘Rana box’, which was confirmed to have no impact on antimicrobial activity. The results of antimicrobial activity assessment against seven microorganisms demonstrated GL-22 to have a broad-spectrum antimicrobial activity, but weak potency against Candida albicans (C. albicans). These data were similar to those of GL-29, but GL-22 showed much lower haemolysis and lower cytotoxicity against HaCaT cells. Moreover, GL-22 exhibited potent in vivo activity at 4 × MIC against Staphylococcus aureus (S. aureus)-infected larvae. Several short analogues, from the C-terminus and N-terminus of GL-22, were modified to identify the shortest functional motif. However, the results demonstrated that the shorter peptides did not exhibit potent antimicrobial activity, and the factors that affect the bioactive potency of these short analogues need to be further studied.

Antimicrobial and Anti-inflammatory Effects of a Novel Peptide From the Skin of Frog Microhyla pulchra

Brevinins are an important antimicrobial peptide (AMP) family identified in the skin of Ranidae frogs and generally contain a characteristic ranabox structure at their C-terminal sequence. Herein a novel AMP named brevinin-2MP has been identified from the skin of the frog Microhyla pulchra by molecular cloning. Brevinin-2MP (GVITDTLKGVAKTVAAELLRKAHCKLTNSC) with a high amphipathic α-helix in sodium dodecyl sulfate solutions can destroy bacterial cell membrane and kill microbes. Furthermore, brevinin-2MP has been found to inhibit the lipopolysaccharide (LPS)-induced expression of pro-inflammatory NO, MCP-1, IL-6, and TNF-α via binding unidentified targets on the cell membrane and consequently suppressing the activation of MAPK/NF-κB signaling cascades induced by LPS in RAW 264.7 cells. Consistently, brevinin-2MP significantly alleviates the acute inflammatory response in carrageenan-induced mice paw. In conclusion, brevinin-2MP with anti-inflammatory and antimicrobial properties will be an ideal candidate drug molecule for bacterial inflammation treatment.

Brevinin-2GHk, a Peptide Derived from the Skin of Fejervarya limnocharis, Inhibits Zika Virus Infection by Disrupting Viral Integrity

Several years have passed since the Zika virus (ZIKV) pandemic reoccurred in 2015–2016. However, there is still a lack of proved protective vaccines or effective drugs against ZIKV. The peptide brevinin-2GHk (BR2GK), pertaining to the brevinin-2 family of antimicrobial peptides, has been reported to exhibit only weak antibacterial activity, and its antiviral effects have not been investigated. Thus, we analyzed the effect of BR2GK on ZIKV infection. BR2GK showed significant inhibitory activity in the early and middle stages of ZIKV infection, with negligible cytotoxicity. Furthermore, BR2GK was suggested to bind with ZIKV E protein and disrupt the integrity of the envelope, thus directly inactivating ZIKV. In addition, BR2GK can also penetrate the cell membrane, which may contribute to inhibition of the middle stage of ZIKV infection. BR2GK blocked ZIKV E protein expression with an IC₅₀ of 3.408 ± 0.738 μΜ. In summary, BR2GK was found to be a multi-functional candidate and a potential lead compound for further development of anti-ZIKV drugs.

[Antibacterial mechanism of Brevinin-2GHk, an antimicrobial peptide from Fejervarya limnocharis skin]

OBJECTIVE

To investigate the secondary structure, physicochemical properties and antibacterial activity of Brevinin- 2GHk (BR2GK), an antimicrobial peptide from frog skin, and explore its antibacterial mechanism.

METHODS

BR2GK was synthesized, purified with high performance liquid chromatography (HPLC) and identified using mass spectrometry. Circular dichroism was used to determine the secondary structure and physicochemical properties of BR2GK. Two-fold dilution method was used to determine the antibacterial activity of BR2GK, and its antibacterial mechanism was explored using laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM). The hemolytic activity of BR2GK was analyzed in red blood cells. Isothermal titration calorimetry (ITC) and surface plasmon resonance imaging (SPRi) were employed to detect the binding of BR2GK to lipopolysaccharide (LPS), and the antioxidant activity of BR2GK was evaluated using biochemical kits.

RESULTS

The synthesized BR2GK, with a purity exceeding 95% after purification, had the basic characteristics of cationic antimicrobial peptides. BR2GK consisted mainly of α-helical structure in SDS solution and exhibited a broad-spectrum antibacterial activity. Antibacterial activity assay showed that BR2GK had inhibitory and killing activity against a variety of strains with a minimum inhibitory concentration (MIC) of 2.76 μmol/L against Staphylococcus aureus. Observation with LSCM and SEM showed that BR2GK at an active concentration caused bacterial cell membrane damage, cell swelling, contraction, deformation, and massive exudation of intracellular contents without causing hemolysis. ITC showed that the binding affinity K_D of BR2GK to LPS was 18.2±0.8 μmol/L. The antioxidant test showed that BR2GK was capable of effectively scavenging NO, ABTS and DPPH.

CONCLUSION

BR2GK, as a broad-spectrum antibacterial peptide with also a strong antioxidant capacity, does not cause hemolysis and is capable of binding to LPS. BR2GK has an important value for future design and synthesis of antimicrobial peptides with stronger antibacterial activity and lower cytotoxicity.

Bioevaluation and Targeted Modification of Temporin-FL From the Skin Secretion of Dark-Spotted Frog (Pelophylax nigromaculatus)

Bioactive proteins secreted by the granular glands of amphibian skin play a self-defensive role, and exhibit various bioactivities in vitro and in vivo. In light of the severity of the problem of antibiotic resistance for treating infections, many antimicrobial peptides (AMPs) have been developed and applied in clinical microbial treatments. We identified a naturally derived and potent antimicrobial peptide, temporin-FL, obtained from the skin secretion of Pelophylax nigromaculatus via “shotgun” cloning. Two truncated analogues of this peptide were chemically synthesized to explore their structural-functional relationships. The results of a functional evaluation showed that all of the tested AMPs were active against Gram-positive bacteria and fungi and demonstrated antibiofilm activity against methicillin-resistant Staphylococcus aureus (MRSA) but did not have an effect on Gram-negative bacteria. Moreover, temporin-FLa demonstrated a higher level of hydrophobicity and enhanced antimicrobial efficiency, as well as hemolytic activity and cell cytotoxicity than the parent peptide. Temporin-FLb, which evidenced significantly less α-helicity, was less potent against various microbes but exhibited lower cytotoxicity relating to mammalian cells. Both of the synthesized analogues possessed a higher therapeutic index than the original peptide. Moreover, the membrane permeability assay and the measuring membrane depolarization assay declared that temporin-FL and its analogues induced membrane fracture and depolarization; the quantitative biofilm formation assay and the observations of MRSA biofilms using scanning electron microscopy revealed that the AMPs caused biofilm disruption and blocked biofilm formation, the former experiments all confirming their antimicrobial and antibiofilm properties. Hence, the optimization of temporin-FL offers insights for the discovery of new drugs for treating MRSA infections.

Generation of truncated derivatives through in silico enzymatic digest of peptide GV30 target MRSA both in vitro and in vivo

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A novel host-defence peptide GV30 was identified from the frog skin secretion of Hylarana guentheri.

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Seven short AMPs were generated by in silico enzymatic digest of GV30 using an online proteomic bioinformatic tool PeptideCutter in ExPASy server.

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Two truncated products, GV23 and GV21, exhibited an improved antibacterial effect against MRSA in vitro and demonstrated a faster bactericidal effect than the parent peptide.

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GV 21 was found to have a better in vivo anti-MRSA activity and retain the good antibacterial activity under salt and serum conditions, along with lower toxicity.

Methicillin-resistant Staphylococcus aureus (MRSA) causing serious hospital-acquired infections and skin infections has become a “superbug” in clinical treatment. Although the clinical treatment of MRSA is continuously improving, due to its unceasing global spread, MRSA has produced much heated discussion and focused study, therefore suggesting an urgent task to find new antibacterial drugs to combat this issue. Antimicrobial peptides (AMPs) are used as the last-resort drugs for treating multidrug-resistant bacterial infections, but their utilisation is still limited due to their low stability and often strong toxicity.

Here, we evaluated the structure and the bioactivity of an AMP, GV30, derived from the frog skin secretions of Hylarana guentheri, and designed seven truncated derivatives based on the presence of cleavage sites for trypsin using an online proteomic bioinformatic resource PeptideCutter tool. We investigated the anti-MRSA effect, toxicity and salt- and serum-resistance of these peptides. Interestingly, the structure–activity relationship revealed that removing “Rana box” loop could significantly improve the bactericidal speed on MRSA. Among these derivatives, GV21 (GVIFNALKGVAKTVAAQLLKK-NH₂), because of its faster antibacterial effect, lower toxicity, and retains the good antibacterial activity and stability of the parent peptide, is considered to become a new potential antibacterial candidate against MRSA.

Bacterial-Specific Aggregation and Killing of Immunomodulatory Host Defense Peptides

This study involves the design and development of disulfide bridge-linked antimicrobial peptides using the host defense protein Angiogenin 4 (chAng4) as a template. The mini peptides derived from chAng4 (mCA4s) were evaluated for their antibacterial efficacies in various pathogenic bacterial strains, and the role of the oxidation state of thiols in the peptide sequence and its implication on antibacterial properties were explored. A remarkable property of these synthetic mCA4 peptides is their capability to flocculate bacteria and mediate bacterial-specific killing, in the absence of any other external stimulus. mCA4s were further evaluated for their cellular uptake, hemolytic activities, toxicities, and immunomodulatory activities in different eukaryotic cell lines. The results indicate that disulfide bridge-containing cationic amphipathic peptides show superior antibacterial efficacies, are nontoxic and nonhemolytic, and mediate bacterial flocculation and killing, in the absence of external stimuli.

Study on the Structure-Activity Relationship of an Antimicrobial Peptide, Brevinin-2GUb, from the Skin Secretion of Hylarana guentheri

Antimicrobial peptides (AMPs) are considered potential alternatives to antibiotics due to their advantages in solving antibiotic resistance. Brevinin-2GUb, which was extracted from the skin secretion of Hylarana guentheri, is a peptide with modest antimicrobial activity. Several analogues were designed to explore the structure–activity relationship and enhance its activity. In general, the Rana box is not an indispensable motif for the bioactivity of Brevinin-2GUb, and the first to the 19th amino acids at the N-terminal end are active fragments, such that shortening the peptide while maintaining its bioactivity is a promising strategy for the optimisation of peptides. Keeping a complete hydrophobic face and increasing the net charges are key factors for antimicrobial activity. With the increase of cationic charges, α-helical proportion, and amphipathicity, the activity of t-Brevinin-2GUb-6K (tB2U-6K), in combatting bacteria, drastically improved, especially against Gram-negative bacteria, and the peptide attained the capacity to kill clinical isolates and fungi as well, which made it possible to address some aspects of antibiotic resistance. Thus, peptide tB2U-6K, with potent antimicrobial activity against antibiotic-resistant bacteria, the capacity to inhibit the growth of biofilm, and low toxicity against normal cells, is of value to be further developed into an antimicrobial agent.

Characterisation of a novel peptide, Brevinin-1H, from the skin secretion of Amolops hainanensis and rational design of several analogues

As drug-resistant bacteria have become a serious health problem and have caused thousands of deaths, finding new antibiotics has become an urgent research priority. A novel antimicrobial peptide, named Brevinin-1H, was identified in the skin secretion of Amolops hainanensis through 'shotgun' cloning. It has broad-spectrum antimicrobial activity against tested micro-organisms and has anticancer cell activity. To improve its bioactivity and decrease its cytotoxicity, two structural analogues-Brevinin-1Ha and Brevinin-1HY-were designed based on the secondary structure of the natural peptide. Brevinin-1HY, in which tyrosine substituted Pro¹¹ , had similar activity to the natural peptide against Gram-negative bacteria and cancer cells, but showed a dramatic increase in haemolytic activity and cytotoxicity at its minimum inhibitory concentration. Brevinin-1Ha, which transferred the Rana-box from the C-terminal to a central position, had significantly decreased haemolytic activity, but also in antimicrobial and anticancer activity. The present data suggest that increasing the proportion of α-helix structure in an AMP can increase its target micro-organism bioactivity to some extent.