Structural features of helical antimicrobial peptides: their potential to modulate activity on model membranes and biological cells

Advances in antimicrobial peptides: From mechanistic insights to chemical modifications

This review provides a comprehensive analysis of antimicrobial peptides (AMPs), exploring their diverse sources, secondary structures, and unique characteristics. The review explores into the mechanisms underlying the antibacterial, immunomodulatory effects, antiviral, antiparasitic and antitumour of AMPs. Furthermore, it discusses the three principal synthesis pathways for AMPs and assesses their current clinical applications and preclinical research status. The paper also addresses the limitations of AMPs, including issues related to stability, resistance, and toxicity, while offering insights into strategies for their enhancement. Recent advancements in AMP research, such as chemical modifications (including amino acid sequence optimisation, terminal and side-chain modifications, PEGylation, conjugation with small molecules, conjugation with photosensitisers, metal ligands, polymerisation, cyclisation and specifically targeted antimicrobial peptides) are highlighted. The goal is to provide a foundation for the future design and optimisation of AMPs.

Serum-Stable, Cationic, α-Helical AMPs to Combat Infections of ESKAPE Pathogens and C. albicans

Expedition in the rate of development of antimicrobial resistance accompanied by the slowdown in the development of new antimicrobials has led to a dire necessity to develop an alternate class of antimicrobial agents. Antimicrobial peptides (AMPs), available in nature, are effective molecules that can combat microbial infections. However, due to several inherent shortcomings such as salt sensitivity of their potency, short systemic half-lives owing to protease and serum degradation, and cytotoxicity, their commercial success is limited. Inspired by α helical AMPs present in nature, here in this work, we have developed two short, cationic, helical AMPs RR-12 and FL-13. Both peptides exhibited high broad-spectrum antimicrobial activity, salt tolerance, prompt bactericidal activity, considerable serum stability, remaining non-cytotoxic and non-hemolytic at relevant microbicidal concentrations. The designed AMPs were membranolytic toward the microbial strains, though there were subtle differences in the mechanism owing to the variation in the composition of the cell membranes in different microbes. Rigorous experimental techniques and molecular dynamics (MD) simulations were performed to understand the structure, activity, and their mechanisms in detail. Positive charge, balanced hydrophobicity-hydrophilicity, and helical conformation were the different attributes that led to the development of the superior performance of the AMPs, making them valuable additions to the repertoire of therapeutically promising antimicrobials.

Fabrication of N-acetylcysteine-loaded chitosan-cloaked polyphenol nanoparticles for treatment of pediatric pneumonia and acute lung injury

Bacterial infectious acute pneumonia has long presented a significant barrier to human health and the fast elimination of antibacterial in lung tissue. Engineering nanoformulations that are easily prepared and possess mucoadhesive characteristics for administering antibacterial drugs are crucial for addressing pneumonia and lung injury. This investigation utilized FDA-approved tannic acid (TA) to develop a nanocomplex by cloaking chitosan (CH) to attain prolonged anti-infection efficacy against acute pneumonia. The flash nanocomplexation (FNC) process was employed for developing chitosan-cloaked poly(vinyl alcohol)/TA/N-acetylcysteine (NAC) nanoparticles (CPTN NPs) using NAC as the model drug, relying on non-covalent interactions between the components. The investigation of pneumonia revealed that the robust electrostatic interaction between negatively charged mucin and positively charged chitosan in the trachea facilitated the retention of NAC in the lungs for a minimum of 24 h post-inhalation of CPTN NPs, effectively constraining pneumonia within 3 days. The DPPH values of 97.42 ± 5.1 for CPTN NPs reveal excellent antioxidant ability. The cell viability of NCI-H441 and A549 cells remained above 90% of 100 μg/mL for NAC and CPTN NPs. The antibacterial efficacy of CPTN NPs exhibited a 99.9% reduction compared to the untreated group. The mucoadhesive CPTN NPs, characterized by excellent biocompatibility and produced using a simple and reproducible method, may offer a novel approach to administering CPTN NPs to address acute pediatric pneumonia and lung injury.

Terminally Symmetric β-Turn Peptides for Multidrug-Resistant Bacterial Infections

Antimicrobial peptides (AMPs) are considered promising agents to solve the problem of antibiotic resistance due to their unique membrane-disruption mechanism. In this research, de novo terminally symmetric β-turn AMPs were designed by combining the β-turn sequences derived from Tritrpticin with alternately arranged cationic and hydrophobic amino acid sequences. The structure-activity relationship of the peptides was studied. Among the designed peptides, P-07 (KIKIKPWWWPKIKIK-NH2) exhibited potent antimicrobial activity against all the tested bacterial strains, showing the highest bacterial selectivity, relatively low cytotoxicity, high bactericidal efficiency, and low potential to induce bacterial resistance. The antimicrobial mechanisms of P-07 involving membrane-disruption and lipopolysaccharide-binding were proven. Moreover, the in vivo studies confirmed the wound-healing ability of P-07 using a mice bacteria-infected full-thickness wound model. Taken together, P-07 showed great promise in the treatment of multidrug-resistant bacterial infections.

Evaluation of the antimicrobial efficiency of three novel chimeric peptides through biochemical and biophysical analyses

Three chimeric membrane-active antimicrobial peptides (AMPs) were designed from previously characterized parental molecules, namely pandinin-2, ascaphin-8, and maximin-3. The aim of constructing these chimeras was to obtain sequences with improved therapeutic indices or increased activity, while simultaneously investigating the functional roles of key segments of the parental peptides. Chimera-1 was the most active peptide against clinically relevant bacterial species, followed by chimera-2, and chimera-3, respectively, with no clear preference towards Gram-negative or Gram-positive strains. Escherichia coli and Pseudomonas aeruginosa were the most sensitive bacteria, while Klebsiella pneumoniae and Staphylococcus aureus were resistant to AMP activity. All peptides presented significantly lower activities towards human erythrocytes, with chimera-1 being the most selective. Additionally, only chimera-2 showed cytotoxicity towards Vero cells. Calcein leakage and dynamic light scattering assays using liposomal formulations indicated that the chimeras conserved the pore forming membrane perturbation mechanisms of the parental molecules. Peptide interaction also reduced membrane fluidity. Circular dichroism (CD) data showed disordered peptides in aqueous solution that transitioned into alpha helical structures lipid bilayer environments. In silico assessments correlated well with microbiological and in vitro experimental data. All peptides established greater contact with the bacterial biomimetic membrane compared to the erythrocyte system, as analyzed by distance from membrane surface, number of contacts, solvent accessible surface area, and number of hydrogen bonds. Additionally, the presence of the bilayer lipid patches favored peptide folding, consistent with CD experiments. Molecular dynamics simulations of peptide aggregation revealed that chimera-2 formed the largest oligomers, consistent with the predicted aggregation propensities and the predicted physico-chemical properties. Interaction with membrane surfaces resulted in smaller clusters while low or lack of interaction favored larger aggregates. Overall, the chimeric peptides displayed higher activity and selectivity compared to the parental ones. The contribution of the flanking regions of pandidin-2 and maximin-3 with respect to the core region of ascaphin-8 was not clear.

Self-Assembled Short Peptide Amphiphile-Gold Nanostructures: A Novel Approach for Bacterial Infection Treatment

This study presents a simple and effective strategy for synthesizing biocompatible hybrid nanostructures composed of short peptide amphiphiles (sPA) and gold nanoparticles (AuNPs) for bacterial infection control. The self-assembling sPA molecules form stable β-sheet structures, which are further enhanced upon the addition of gold ions (Au(III)) and brief sunlight exposure, leading to the formation of functional AuNP-sPA nanostructures. Comprehensive spectroscopic and microscopic characterization confirms the successful integration of AuNPs with sPA, resulting in stable nanomaterials with potent antibacterial properties. The AuNP-sPA conjugates exhibit superior antibacterial activity against Gram-negative bacteria, including Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa with a high bacterial selectivity and biocompatibility with minimal cytotoxicity, reinforcing their therapeutic potential. These findings highlight AuNP-sPA nanostructures as promising alternatives to conventional antibiotics for targeted bacterial infection treatment.

Multi-active phlorotannins boost antimicrobial peptide LL-37 to promote periodontal tissue regeneration in diabetic periodontitis

The bidirectional correlation between diabetes and periodontitis positions the latter as the most prevalent complication of the former. Rehabilitation of the periodontal tissues damaged by diabetic periodontitis presents a significant clinical challenge. The multifaceted nature of the pathogenesis of diabetic periodontitis necessitates a comprehensive approach in its treatment to mitigate its adverse effects. To address this, a temperature-sensitive hydrogel containing phlorotannins (PL) and antimicrobial peptide LL-37 was developed to shift the microenvironment of diabetic periodontitis from an exacerbated high-glycemic inflammatory state to a regenerative one. The addition of PL significantly enhanced the antimicrobial properties, stability, and safety of LL-37. Vitro experiments confirmed that PL/LL-37 had good biocompatibility and promoted osteogenic differentiation of bone. PL/LL-37 demonstrated antioxidant properties by scavenging DPPH free radicals and inhibiting NO production. Furthermore, PL/LL-37 effectively modulated macrophage polarization from a M1 phenotype to an M2 phenotype through NF-κB P-p65 inflammatory pathway, thereby reducing the release of pro-inflammatory cytokines and promoting the secretion of anti-inflammatory cytokines. Interestingly, it could downregulate the AGE-RAGE signaling pathway, exerting a protective effect against diabetes. In addition, PL/LL-37 could attenuate inflammation levels, inhibit osteoclast production, promote bone regeneration, inhibit apoptosis and decrease RAGE levels in a rat model of diabetic periodontitis. These combined features synergistically accelerate diabetic periodontal bone regeneration. Consequently, PL/LL-37 emerges as a promising candidate for clinical treatment of diabetic periodontitis.

Surface charge of the C-terminal helix is crucial for antibacterial activity of endolysin against Gram-negative bacteria

BACKGROUNDS

Endolysins are promising alternatives to antibiotics because they can lyse bacterial cells rapidly with a low risk of resistance development, however, their effectiveness against Gram-negative bacteria is hindered by the presence of the outer membrane present in Gram-negative bacteria. Several endolysins with amphipathic helices at the C-terminus have been reported to have intrinsic antibacterial activity against Gram-negative bacteria but their action mechanism is not fully elucidated.

METHODS

The sequence alignment analysis was assessed with the CLC Main workbench 7, and His-tagged endolysins were purified with affinity chromatography. Site-directed mutagenesis was used to generate mutations in the endolysin to make various endolysin mutants. The muralytic activity of the endolysin against Gram-negative bacteria was analyzed using a turbidity reduction assay and the antibacterial activities of the endolysins were assessed through a viable cell counting assay.

RESULTS

We identified two endolysins, LysTS3 and LysTS6, both of which have similar sequences and structures including the amphipathic helices at their C-terminus. LysTS6 exhibited significantly higher antibacterial activity against Gram-negative bacteria compared to LysTS3 even though both enzymes have similar muralytic activity against the outer membrane-permeabilized Gram-negative bacteria. Systematic truncation and bioinformatic analysis of these two endolysins revealed a major difference in the charge on the surface of their C-terminal helices, suggesting the possibility that the charge on this helix can determine the antibacterial activity of the endolysins against Gram-negative bacteria. We could enhance the activity of LysTS3 against Gram-negative bacteria by replacing Ala156 and Glu160 with lysine and alanine, respectively, the amino acid residues at the structurally equivalent positions in LysTS6. A similar activity boost was also seen in LysSPN1S and LysJEP4 when the surface charge of the C-terminal amphipathic helix was altered to be more positive through the modification of the surface-exposed amino acid residues.

CONCLUSIONS

The antibacterial activity of endolysin against Gram-negative bacteria could be enhanced by adjusting the surface charge on the C-terminal amphipathic helix to more positive, suggesting that the positive surface charge on the C-terminal amphipathic helix of endolysin is crucial for its penetration of outer membrane to reach peptidoglycan layer of Gram-negative bacteria.

Effects of Surface Charge of Amphiphilic Peptides on Peptide-Lipid Interactions in the Gas Phase and in Solution

The interactions between peptides and lipids are fundamental for many biological processes. Therefore, exploring the noncovalent interactions that govern these interactions has become increasingly important. Native mass spectrometry is a valuable technique for the characterization of specific peptide-lipid interactions. However, native mass spectrometry requires the transfer of the analyte into the gas phase, and noncovalent interactions driven by the hydrophobic effect might be distorted. We, therefore, address the importance of electrostatic interactions for the formation of peptide-lipid interactions. For this, we make use of the amphipathic, antimicrobial peptide LL-37 as well as a positively and a negatively charged variant thereof and study binding of a variety of lipids by native mass spectrometry. We found that the surface charge of the peptides affects the transfer of stable peptide-lipid complexes into the gas phase and that the ionization mode is important to observe these interactions. We further compare our findings observed in the gas phase with interactions formed in solution between the peptides and lipid monolayers using a Langmuir film balance. The two approaches deliver comparable results and reveal a clear trend in the lipid preferences of all variants for those lipids with opposite charge. Notably, the unmodified wild-type peptide was more flexible in the formation of peptide-lipid interactions. We conclude that native mass spectrometry is indeed well-suited to explore the interactions between peptides and lipids and that electrostatic interactions as expressed by the surface charge of the peptides play an important role in the formation and stabilization of peptide-lipid interactions.

Proline-Modified (RW)n Peptides: Enhancing the Antimicrobial Efficacy and Selectivity against Multidrug-Resistant Pathogens

The growing threat of multidrug-resistant (MDR) bacteria necessitates the development of novel antimicrobial agents. This study investigates the potential of proline-modified (RW)n peptides as a platform for combating MDR pathogens with minimal toxicity. We synthesized and evaluated (RW)n peptides (n = 4, 6, and 8) with and without central proline residues against five clinically relevant bacterial strains, including ESKAPE pathogens. Antimicrobial activity, cytotoxicity, and synergistic effects with conventional antibiotics were assessed. Proline incorporation significantly enhanced the peptide selectivity and broadened the spectrum of activity, particularly against Gram-negative bacteria. RW6P and RW8P demonstrated exceptional efficacy (MICs ≤ 0.25 μg/mL) against methicillin-resistant Staphylococcus aureus and Escherichia coli with minimal toxicity to human cells. Notably, RW8P restored ampicillin susceptibility in Pseudomonas aeruginosa (MIC < 0.25 μg/mL) without dose-dependent toxicity. Exceptionally, RW6-2P demonstrated efficacy against all Gram-positive and Gram-negative bacteria, except Klebsiella pneumoniae (Kp), without toxicity. Furthermore, several Gram-negative isolates were rendered susceptible to vancomycin when combined with these peptides, addressing a key limitation of glycopeptide antibiotics. Gram-negative Klebsiella pneumoniae (Kp) was rendered susceptible to vancomycin when combined with RW4P and RW6-2P. RW4P and RW6-2P, with and without antibiotics, have shown selectivity. This study presents proline-modified (RW)n peptides as promising candidates for developing broad-spectrum antimicrobials with enhanced selectivity and the potential to revitalize existing antibiotics against MDR pathogens.

Harnessing bacterial antimicrobial peptides: a comprehensive review on properties, mechanisms, applications, and challenges in combating antimicrobial resistance

Antimicrobial resistance (AMR) is a significant global health concern due to the persistence of pathogens and the emergence of resistance in bacterial infections. Bacterial-derived antimicrobial peptides (BAMPs) have emerged as a promising strategy to combat these challenges. Known for their diversity and multifaceted nature, BAMPs are notable bioactive agents that exhibit potent antimicrobial activities against various pathogens. This review explores the intricate properties and underlying mechanisms of BAMPs, emphasizing their diverse applications in addressing AMR. Additionally, the review investigates the mechanisms, analyses the challenges in utilizing BAMPs effectively, and examines their potential applications and associated deployment challenges providing comprehensive insights into how BAMPs can be harnessed to combat AMR across different domains. The significance of this review lies in highlighting the potential of BAMPs as transformative agents in combating AMR, offering sustainable and eco-friendly solutions to this pressing global health challenge.

Trp residues near peptide termini enhance the membranolytic activity of cationic amphipathic α-helices

KIA peptides were designed as a series of cationic antimicrobial agents of different lengths, based on the repetitive motif [KIAGKIA]. As amphiphilic helices, they tend to bind initially to the surface of lipid membranes. Depending on the conditions, they are proposed to flip, insert and form toroidal pores, such that the peptides are aligned in a transmembrane orientation. Tryptophan residues are often found near the ends of transmembrane helices, anchoring them to the amphiphilic bilayer interfaces. Hence, we introduced Trp residues near one or both termini of KIA peptides with lengths of 14-24 amino acids. Our hypothesis was that if Trp residues can stabilize the transmembrane orientation, then these KIA peptides will exhibit an increased propensity to form pores, with increased membranolytic activity. Using solid-state 15N NMR, we found that peptides with Trp near the ends are indeed more likely to be flipped into a transmembrane orientation, especially short peptides. Short KIA peptides also exhibited higher antimicrobial activity when modified with Trp, while longer peptides showed similar activities with and without Trp. The hemolytic activity of KIA peptides of all lengths was higher with Trp near the ends. Vesicle leakage was also increased (sometimes more than 10-fold) for the Trp-mutants, especially in thicker membranes. Higher functionality of amphiphilic helices may thus be achieved in general by exploiting the anchoring effect of Trp. These results demonstrate that the incorporation of Trp increases membranolytic activities (vesicle leakage, hemolysis and antimicrobial activity), in a way compatible with a transmembrane pore model of peptide activity.

Dithiocarbamate-Based Sequence-Defined Oligomers as Promising Membrane-Disrupting Antibacterial Agents: Design, Activity, and Mechanism

The emerging prevalence of antimicrobial resistance demands cutting-edge therapeutic agents to treat bacterial infections. We present a synthetic strategy to construct sequence-defined oligomers (SDOs) by using dithiocarbamate (DTC). The antibacterial activity of the synthesized library of SDOs was studied using a Gram-positive B. subtilis and a Gram-negative E. coli. Among SDOs, Dec (with C10 aliphatic chain) was found to be the most promising antibacterial agent exhibiting a minimum inhibitory concentration (MIC) of 3 μg/mL against B. subtilis. Structure-activity relationship studies led to a 400-fold improvement in the MIC within the SDO library. The mode of action of the SDOs was elucidated on a model system, where bacterial membranes mimicking giant unilamellar vesicles (GUVs) were exposed to the SDOs. Membrane disruption and pore formation were found to be the key mechanisms through which SDOs act. In addition, scanning electron microscopy (SEM) and confocal laser scanning microscopy analysis of Dec-treated bacteria confirmed the loss of cell membrane integrity. Finally, the hemolysis assay with SDOs revealed their excellent selectivity toward bacterial cells. Taken together, we developed a modular platform for the synthesis of SDOs having promising antibacterial activity and superior selectivity toward bacteria, with the membrane disruption mode of action confirmed via studies on the model GUV system and SEM analysis.

Antimicrobial peptides: An alternative strategy to combat antimicrobial resistance

Antimicrobial peptides (AMPs) are a diverse group of naturally occurring molecules produced by eukaryotes and prokaryotes. They have an important role in innate immunity via their direct microbicidal properties or immunomodulatory activities against pathogens. With the widespread occurrence of antimicrobial resistance (AMR), AMPs are considered as viable alternatives for the treatment of multidrug-resistant microbes, inflammation, and, wound healing. The broad-spectrum antibacterial activity of AMPs is predominantly attributed to membrane disruption, leading to the formation of transmembrane pores and, eventually, cell lysis. However, mechanisms related to inhibition of cell wall synthesis, nucleic acid synthesis, protein synthesis, or enzymatic activity are also associated with these peptides. In this review, we discuss our current understanding, therapeutic uses and challenges associated with the clinical applications of AMPs.

A Zeolitic Imidazolate Framework-Based Antimicrobial Peptide Delivery System with Enhanced Anticancer Activity and Low Systemic Toxicity

BACKGROUND

The clinical efficacies of anticancer drugs are limited by non-selective toxic effects on healthy tissues and low bioavailability in tumor tissue. Therefore, the development of vehicles that can selectively deliver and release drugs at the tumor site is critical for further improvements in patient survival.

METHODS

We prepared a CEC nano-drug delivery system, CEC@ZIF-8, with a zeolite imidazole framework-8 (ZIF-8) as a carrier, which can achieve the response of folate receptor (FR). We characterized this system in terms of morphology, particle size, zeta potential, infrared (IR), x-ray diffraction (XRD), and transcriptome analysis, and examined the in vitro cytotoxicity and cellular uptake properties of CEC@ZIF-8 using cervical cancer cells. Lastly, we established a TC-1 tumor-bearing mouse model and evaluated its in vivo anti-cervical cancer activity.

RESULTS

The CEC@ZIF-8 nano-delivery system had favorable biocompatibility, heat stability, and pH responsiveness, with a CEC loading efficiency of 12%, a hydrated particle size of 174 ± 5.8 nm, a zeta potential of 20.57 mV, and slow and massive drug release in an acidic environment (pH 5.5), whereas release was 6% in a neutral environment (pH 7.4). At the same time, confocal imaging and cell viability assays demonstrated greater intracellular accumulation and more potent cytotoxicity against cancer cells compared to free CEC. The mechanism was analyzed by a series of transcriptome analyses, which revealed that CEC@ZIF-8 NPs differentially regulate the expression levels of 1057 genes in cancer cells, and indicated that the enriched pathways were mainly cell cycle and apoptosis-related pathways via the enrichment analysis of the differential genes. Flow cytometry showed that CEC@ZIF-8 NPs inhibited the growth of HeLa cells by arresting the cell cycle at the G0/G1 phase. Flow cytometry also revealed that CEC@ZIF-8 NPs induced greater apoptosis rates than CEC, while unloaded ZIF-8 had little inherent pro-apoptotic activity. Furthermore, the levels of reactive oxygen species (ROS) were also upregulated by CEC@ZIF-8 NPs while ROS inhibitors and caspase inhibitors reversed CEC@ZIF-8 NPs-induced apoptosis. Finally, CEC@ZIF-8 NPs also reduced the growth rate of xenograft tumors in mice without the systemic toxicity observed with cisplatin treatment.

CONCLUSIONS

The CEC@ZIF-8 nano-drug delivery system significantly enhanced the anti-cervical cancer effect of CEC both in vivo and in vitro, providing a more promising drug delivery system for clinical applications and tumor management. At the same time, this work demonstrates the clinical potential of CEC-loaded ZIF-8 nanoparticles for the selective destruction of tumor tissues.

Boosting the antibacterial potential of a linear encrypted peptide in a Kunitz-type inhibitor (ApTI) through physicochemical-guided approaches

Bacterial resistance has become a serious public health problem in recent years, thus encouraging the search for new antimicrobial agents. Here, we report an antimicrobial peptide (AMP), called PEPAD, which was designed based on an encrypted peptide from a Kunitz-type plant peptidase inhibitor. PEPAD was capable of rapidly inhibiting and eliminating numerous bacterial species at micromolar concentrations (from 4μM to 10 μM), with direct membrane activity. It was also observed that the peptide can act synergistically with ciprofloxacin and showed no toxicity in the G. mellonella in vivo assay. Circular dichroism assays revealed that the peptide's secondary structure adopts different scaffolds depending on the environment in which it is inserted. In lipids mimicking bacterial cell membranes, PEPAD adopts a more stable α-helical structure, which is consistent with its membrane-associated mechanism of action. When in contact with lipids mimicking mammalian cells, PEPAD adopts a disordered structure, losing its function and suggesting cellular selectivity. Therefore, these findings make PEPAD a promising candidate for future antimicrobial therapies with low toxicity to the host.

Flexible fluorine-thiol displacement stapled peptides with enhanced membrane penetration for the estrogen receptor/coactivator interaction

Understanding how natural and engineered peptides enter cells would facilitate the elucidation of biochemical mechanisms underlying cell biology and is pivotal for developing effective intracellular targeting strategies. In this study, we demonstrate that our peptide stapling technique, fluorine-thiol displacement reaction (FTDR), can produce flexibly constrained peptides with significantly improved cellular uptake, particularly into the nucleus. This platform confers enhanced flexibility, which is further amplified by the inclusion of a D-amino acid, while maintaining environment-dependent α helicity, resulting in highly permeable peptides without the need for additional cell-penetrating motifs. Targeting the estrogen receptor α (ERα)-coactivator interaction prevalent in estrogen receptor-positive (ER+) breast cancers, we showcased that FTDR-stapled peptides, notably SRC2-LD, achieved superior internalization, including cytoplasmic and enriched nuclear uptake, compared to peptides stapled by ring-closing metathesis. These FTDR-stapled peptides use different mechanisms of cellular uptake, including energy-dependent transport such as actin-mediated endocytosis and macropinocytosis. As a result, FTDR peptides exhibit enhanced antiproliferative effects despite their slightly decreased target affinity. Our findings challenge existing perceptions of cell permeability, emphasizing the possibly incomplete understanding of the structural determinants vital for cellular uptake of peptide-like macromolecules. Notably, while α helicity and lipophilicity are positive indicators, they alone are insufficient to determine high-cell permeability, as evidenced by our less helical, more flexible, and less lipophilic FTDR-stapled peptides.

Antibacterial Activity of AXOTL-13, a Novel Peptide Identified from the Transcriptome of the Salamander Ambystoma mexicanum

Background/Objectives: Antimicrobial peptides are essential molecules in the innate immunity of various organisms and possess a broad spectrum of antimicrobial, antitumor, and immunomodulatory activities. Due to their multifunctionality, they are seen as an alternative for controlling bacterial infections. Although conventional antibiotics have improved health worldwide, their indiscriminate use has led to the emergence of resistant microorganisms. To discover new molecules with antimicrobial activity that could overcome the limitations of traditional antibiotics, this study aimed to identify antimicrobial peptides in Ambystoma mexicanum. Methods: In this study, hypothetical proteins encoded in the Ambystoma mexicanum transcriptome were predicted. These proteins were aligned with peptides reported in the Antimicrobial Peptide Database (APD3) using the Fasta36 program. After identifying peptide sequences with potential antibacterial activity, their expression was confirmed through conventional polymerase chain reaction (PCR) and then chemically synthesized. The antibacterial activity of the synthesized peptides was evaluated against Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC 25922. Results: A new antimicrobial peptide named AXOTL-13 was identified. AXOTL-13 is an amphipathic cationic alpha-helical peptide with the ability to inhibit the growth of Escherichia coli without causing hemolysis in red blood cells, with its action likely directed at the membrane, as suggested by morphological changes observed through scanning electron microscopy. Conclusions: This research is pioneering in evaluating the activity of antimicrobial peptides present in Ambystoma mexicanum and in specifically identifying one of these peptides. The findings will serve as a reference for future research in this field.

Molecular and energetic analysis of the interaction and specificity of Maximin 3 with lipid membranes: In vitro and in silico assessments

In this study, the interaction of antimicrobial peptide Maximin 3 (Max3) with three different lipid bilayer models was investigated to gain insight into its mechanism of action and membrane specificity. Bilayer perturbation assays using liposome calcein leakage dose-response curves revealed that Max3 is a selective membrane-active peptide. Dynamic light scattering recordings suggest that the peptide incorporates into the liposomal structure without producing a detergent effect. Langmuir monolayer compression assays confirmed the membrane inserting capacity of the peptide. Attenuated total reflection-Fourier transform infrared spectroscopy showed that the fingerprint signals of lipid phospholipid hydrophilic head groups and hydrophobic acyl chains are altered due to Max3-membrane interaction. On the other hand, all-atom molecular dynamics simulations (MDS) of the initial interaction with the membrane surface corroborated peptide-membrane selectivity. Peptide transmembrane MDS shed light on how the peptide differentially modifies lipid bilayer properties. Molecular mechanics Poisson-Boltzmann surface area calculations revealed a specific electrostatic interaction fingerprint of the peptide for each membrane model with which they were tested. The data generated from the in silico approach could account for some of the differences observed experimentally in the activity and selectivity of Max3.

A Review of Antimicrobial Peptides: Structure, Mechanism of Action, and Molecular Optimization Strategies

Antimicrobial peptides (AMPs) are bioactive macromolecules that exhibit antibacterial, antiviral, and immunomodulatory functions. They come from a wide range of sources and are found in all forms of life, from bacteria to plants, vertebrates, and invertebrates, and play an important role in controlling the spread of pathogens, promoting wound healing and treating tumors. Consequently, AMPs have emerged as promising alternatives to next-generation antibiotics. With advancements in systems biology and synthetic biology technologies, it has become possible to synthesize AMPs artificially. We can better understand their functional activities for further modification and development by investigating the mechanism of action underlying their antimicrobial properties. This review focuses on the structural aspects of AMPs while highlighting their significance for biological activity. Furthermore, it elucidates the membrane targeting mechanism and intracellular targets of these peptides while summarizing molecular modification approaches aimed at enhancing their antibacterial efficacy. Finally, this article outlines future challenges in the functional development of AMPs along with proposed strategies to overcome them.

How Unnatural Amino Acids in Antimicrobial Peptides Change Interactions with Lipid Model Membranes

This study investigates the potential of antimicrobial peptides (AMPs) as alternatives to combat antibiotic resistance, with a focus on two AMPs containing unnatural amino acids (UAAs), E2-53R (16 AAs) and LE-54R (14 AAs). In both peptides, valine is replaced by norvaline (Nva), and tryptophan is replaced by 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic). Microbiological studies reveal their potent activity against both Gram-negative (G(-)) and Gram-positive (G(+)) bacteria without any toxicity to eukaryotic cells at test concentrations up to 32 μM. Circular dichroism (CD) spectroscopy indicates that these peptides maintain α-helical structures when interacting with G(-) and G(+) lipid model membranes (LMMs), a feature linked to their efficacy. X-ray diffuse scattering (XDS) demonstrates a softening of G(-), G(+) and eukaryotic (Euk33) LMMs and a nonmonotonic decrease in chain order as a potential determinant for bacterial membrane destabilization. Additionally, XDS finds a significant link between both peptides' interfacial location in G(-) and G(+) LMMs and their efficacy. Neutron reflectometry (NR) confirms the AMP locations determined using XDS. Lack of toxicity in eukaryotic cells may be related to their loss of α-helicity and their hydrocarbon location in Euk33 LMMs. Both AMPs with UAAs offer a novel strategy to wipe out antibiotic-resistant strains while maintaining human cells. These findings are compared with previously published data on E2-35, which consists of the natural amino acids arginine, tryptophan, and valine.

Enhancing Antimicrobial Peptide Activity through Modifications of Charge, Hydrophobicity, and Structure

Antimicrobial peptides (AMPs) are emerging as a promising alternative to traditional antibiotics due to their ability to disturb bacterial membranes and/or their intracellular processes, offering a potential solution to the growing problem of antimicrobial resistance. AMP effectiveness is governed by factors such as net charge, hydrophobicity, and the ability to form amphipathic secondary structures. When properly balanced, these characteristics enable AMPs to selectively target bacterial membranes while sparing eukaryotic cells. This review focuses on the roles of positive charge, hydrophobicity, and structure in influencing AMP activity and toxicity, and explores strategies to optimize them for enhanced therapeutic potential. We highlight the delicate balance between these properties and how various modifications, including amino acid substitutions, peptide tagging, or lipid conjugation, can either enhance or impair AMP performance. Notably, an increase in these parameters does not always yield the best results; sometimes, a slight reduction in charge, hydrophobicity, or structural stability improves the overall AMP therapeutic potential. Understanding these complex interactions is key to developing AMPs with greater antimicrobial activity and reduced toxicity, making them viable candidates in the fight against antibiotic-resistant bacteria.

dsAMP and dsAMPGAN: Deep Learning Networks for Antimicrobial Peptides Recognition and Generation

Antibiotic resistance is a growing public health challenge. Antimicrobial peptides (AMPs) effectively target microorganisms through non-specific mechanisms, limiting their ability to develop resistance. Therefore, the prediction and design of new AMPs is crucial. Recently, deep learning has spurred interest in computational approaches to peptide drug discovery. This study presents a novel deep learning framework for AMP classification, function prediction, and generation. We developed discoverAMP (dsAMP), a robust AMP predictor using CNN Attention BiLSTM and transfer learning, which outperforms existing classifiers. In addition, dsAMPGAN, a Generative Adversarial Network (GAN)-based model, generates new AMP candidates. Our results demonstrate the superior performance of dsAMP in terms of sensitivity, specificity, Matthew correlation coefficient, accuracy, precision, F1 score, and area under the ROC curve, achieving >95% classification accuracy with transfer learning on a small dataset. Furthermore, dsAMPGAN successfully synthesizes AMPs similar to natural ones, as confirmed by comparisons of physical and chemical properties. This model serves as a reliable tool for the identification of novel AMPs in clinical settings and supports the development of AMPs to effectively combat antibiotic resistance.

Targeting Leishmania Promastigotes and Amastigotes Forms through Amino Acids and Peptides: A Promising Therapeutic Strategy

Millions of people worldwide are affected by leishmaniasis, caused by the Leishmania parasite. Effective treatment is challenging due to the biological complexity of the parasite, drug toxicity, and increasing resistance to conventional drugs. To combat this disease, the development of specific strategies to target and selectively eliminate the parasite is crucial. This Review highlights the importance of amino acids in the developmental stages of Leishmania as a factor determining whether the infection progresses or is suppressed. It also explores the use of peptides as alternatives in parasite control and the development of novel targeted treatments. While these strategies show promise for more effective and targeted treatment, further studies to address the remaining challenges are imperative.

A Critical Review of Short Antimicrobial Peptides from Scorpion Venoms, Their Physicochemical Attributes, and Potential for the Development of New Drugs

Scorpion venoms have proven to be excellent sources of antimicrobial agents. However, although many of them have been functionally characterized, they remain underutilized as pharmacological agents, despite their evident therapeutic potential. In this review, we discuss the physicochemical properties of short scorpion venom antimicrobial peptides (ssAMPs). Being generally short (13-25 aa) and amidated, their proven antimicrobial activity is generally explained by parameters such as their net charge, the hydrophobic moment, or the degree of helicity. However, for a complete understanding of their biological activities, also considering the properties of the target membranes is of great relevance. Here, with an extensive analysis of the physicochemical, structural, and thermodynamic parameters associated with these biomolecules, we propose a theoretical framework for the rational design of new antimicrobial drugs. Through a comparison of these physicochemical properties with the bioactivity of ssAMPs in pathogenic bacteria such as Staphylococcus aureus or Acinetobacter baumannii, it is evident that in addition to the net charge, the hydrophobic moment, electrostatic energy, or intrinsic flexibility are determining parameters to understand their performance. Although the correlation between these parameters is very complex, the consensus of our analysis suggests that there is a delicate balance between them and that modifying one affects the rest. Understanding the contribution of lipid composition to their bioactivities is also underestimated, which suggests that for each peptide, there is a physiological context to consider for the rational design of new drugs.

Musical protein: Mapping the time sequence of music onto the spatial architecture of proteins

BACKGROUND AND OBJECTIVE

Music, the ubiquitous language across human cultures, is traditionally considered as a form of art but has been linked to biomolecules in recent years. However, previous efforts have only been addressed on sonification of nucleic acids and proteins to produce so-called life music, the soundscape from the basic building blocks of life. In this study, we attempted to, for the first time, conduct a reverse operation of this process, i.e. conversion of music to protein (CoMtP).

METHODS

A novel notion termed musical protein (MP) -- the protein defined by music -- was proposed and, on this basis, we described a computational strategy to map the time sequence of music onto the spatial architecture of proteins, which considered that each note in the stave of a music (target) can be simply characterized by two acoustical quantities and that each residue in the primary sequence of a protein (hit) was represented by amino acid descriptors.

RESULTS

A simulated annealing (SA) algorithm was applied to iteratively generate the best matched MP hit for a music target and structural bioinformatics was then used to model spatial advanced structure for the resulting MP. We also demonstrated that some small MPs derived from music segments may have potential biological functions, which, for example, can serve as antimicrobial peptides (AMPs) to inhibit clinical bacterial strains with moderate or high antibacterial potency.

CONCLUSIONS

This work may benefit many aspects; for example, it would open a door for the hearing-impaired persons to 'listen' music in a biological vision and could be a mean of exposing students to the concepts of biomolecules at an earlier age through the use of auditory characteristics. The CoMtP would also facilitate the rational design of proteins with biological and medicinal significance.

Development of a Selective and Stable Antimicrobial Peptide

Antimicrobial peptides (AMPs) are presented as potential scaffolds for antibiotic development due to their desirable qualities including broad-spectrum activity, rapid action, and general lack of susceptibility to current resistance mechanisms. However, they often lose antibacterial activity under physiological conditions and/or display mammalian cell toxicity, which limits their potential use. Identification of AMPs that overcome these barriers will help develop rules for how this antibacterial class can be developed to treat infection. Here we describe the development of our novel synthetic AMP, from discovery through in vivo application. Our evolved AMP, DTr18-dab, has broad-spectrum antibacterial activity and is nonhemolytic. It is active against planktonic bacteria and biofilm, is unaffected by colistin resistance, and importantly is active in both human serum and a Galleria mellonella infection model. Several modifications, including the incorporation of noncanonical amino acids, were used to arrive at this robust sequence. We observed that the impact on antibacterial activity with noncanonical amino acids was dependent on assay conditions and therefore not entirely predictable. Overall, our results demonstrate how a relatively weak lead can be developed into a robust AMP with qualities important for potential therapeutic translation.

Non-canonical amino acid bioincorporation into antimicrobial peptides and its challenges

The rise of antimicrobial resistance and multi-drug resistant pathogens has necessitated explorations for novel antibiotic agents as the discovery of conventional antibiotics is becoming economically less viable and technically more challenging for biopharma. Antimicrobial peptides (AMPs) have emerged as a promising alternative because of their particular mode of action, broad spectrum and difficulty that microbes have in becoming resistant to them. The AMPs bacitracin, gramicidin, polymyxins and daptomycin are currently used clinically. However, their susceptibility to proteolytic degradation, toxicity profile, and complexities in large-scale manufacture have hindered their development. To improve their proteolytic stability, methods such as integrating non-canonical amino acids (ncAAs) into their peptide sequence have been adopted, which also improves their potency and spectrum of action. The benefits of ncAA incorporation have been made possible by solid-phase peptide synthesis. However, this method is not always suitable for commercial production of AMPs because of poor yield, scale-up difficulties, and its non-'green' nature. Bioincorporation of ncAA as a method of integration is an emerging field geared towards tackling the challenges of solid-phase synthesis as a green, cheaper, and scalable alternative for commercialisation of AMPs. This review focusses on the bioincorporation of ncAAs; some challenges associated with the methods are outlined, and notes are given on how to overcome these challenges. The review focusses particularly on addressing two key challenges: AMP cytotoxicity towards microbial cell factories and the uptake of ncAAs that are unfavourable to them. Overcoming these challenges will draw us closer to a greater yield and an environmentally friendly and sustainable approach to make AMPs more druggable.

Poptosis or Peptide-Induced Transmembrane Pore Formation: A Novel Way to Kill Cancer Cells without Affecting Normal Cells

Recent advances in cancer treatment like personalized chemotherapy and immunotherapy are aimed at tumors that meet certain specifications. In this review, we describe a new approach to general cancer treatment, termed peptide-induced poptosis, in which specific peptides, e.g., PNC-27 and its shorter analogue, PNC-28, that contain the segment of the p53 transactivating 12-26 domain that bind to HDM-2 in its 1-109 domain, bind to HDM-2 in the membranes of cancer cells, resulting in transmembrane pore formation and the rapid extrusion of cancer cell contents, i.e., tumor cell necrosis. These peptides cause tumor cell necrosis of a wide variety of solid tissue and hematopoietic tumors but have no effect on the viability and growth of normal cells since they express at most low levels of membrane-bound HDM-2. They have been found to successfully treat a highly metastatic pancreatic tumor as well as stem-cell-enriched human acute myelogenous leukemias in nude mice, with no evidence of off-target effects. These peptides also are cytotoxic to chemotherapy-resistant cancers and to primary tumors. We performed high-resolution scanning immuno-electron microscopy and visualized the pores in cancer cells induced by PNC-27. This peptide forms 1:1 complexes with HDM-2 in a temperature-independent step, followed by dimerization of these complexes to form transmembrane channels in a highly temperature-dependent step parallel to the mode of action of other membranolytic but less specific agents like streptolysin. These peptides therefore may be effective as general anti-cancer agents.

Structure modification of anoplin for fighting resistant bacteria

The emergence of bacterial resistance has posed a significant challenge to clinical antimicrobial treatment, rendering commonly used antibiotics ineffective. The development of novel antimicrobial agents and strategies is imperative for the treatment of resistant bacterial infections. Antimicrobial peptides (AMPs) are considered a promising class of antimicrobial agents due to their low propensity for resistance and broad-spectrum activity. Anoplin is a small linear α-helical natural antimicrobial peptide that was isolated from the venom of the solitary wasp Anplius samariensis. It exhibits rich biological activity, particularly broad-spectrum antimicrobial activity and low hemolytic activity. Over the past three decades, more than 40 research publications on anoplin have been made available online. This review focuses on the advancements of anoplin in antimicrobial research, encompassing its sources, characterization, antimicrobial activity, influencing factors and structural modifications. The aim is to provide assistances for the development of new antimicrobial agents that can combat bacterial resistance.

Structural and mechanistic basis for RiPP epimerization by a radical SAM enzyme

D-Amino acid residues, found in countless peptides and natural products including ribosomally synthesized and post-translationally modified peptides (RiPPs), are critical for the bioactivity of several antibiotics and toxins. Recently, radical S-adenosyl-L-methionine (SAM) enzymes have emerged as the only biocatalysts capable of installing direct and irreversible epimerization in RiPPs. However, the mechanism underpinning this biochemical process is ill-understood and the structural basis for this post-translational modification remains unknown. Here we report an atomic-resolution crystal structure of a RiPP-modifying radical SAM enzyme in complex with its substrate properly positioned in the active site. Crystallographic snapshots, size-exclusion chromatography-small-angle x-ray scattering, electron paramagnetic resonance spectroscopy and biochemical analyses reveal how epimerizations are installed in RiPPs and support an unprecedented enzyme mechanism for peptide epimerization. Collectively, our study brings unique perspectives on how radical SAM enzymes interact with RiPPs and catalyze post-translational modifications in natural products.

Structural characterization of the antimicrobial peptides myxinidin and WMR in bacterial membrane mimetic micelles and bicelles

Antimicrobial peptides are a promising class of potential antibiotics that interact selectively with negatively charged lipid bilayers. This paper presents the structural characterization of the antimicrobial peptides myxinidin and WMR associated with bacterial membrane mimetic micelles and bicelles by NMR, CD spectroscopy, and molecular dynamics simulations. Both peptides adopt a different conformation in the lipidic environment than in aqueous solution. The location of the peptides in micelles and bicelles has been studied by paramagnetic relaxation enhancement experiments with paramagnetic tagged 5- and 16-doxyl stearic acid (5-/16-SASL). Molecular dynamics simulations of multiple copies of the peptides were used to obtain an atomic level of detail on membrane-peptide and peptide-peptide interactions. Our results highlight an essential role of the negatively charged membrane mimetic in the structural stability of both myxinidin and WMR. The peptides localize predominantly in the membrane's headgroup region and have a noticeable membrane thinning effect on the overall bilayer structure. Myxinidin and WMR show a different tendency to self-aggregate, which is also influenced by the membrane composition (DOPE/DOPG versus DOPE/DOPG/CL) and can be related to the previously observed difference in the ability of the peptides to disrupt different types of model membranes.

Computational modelling of the antimicrobial peptides Cruzioseptin-4 extracted from the frog Cruziohyla calcarifer and Pictuseptin-1 extracted from the frog Boana picturata

A computational study of the peptides Cruzioseptin-4 and Pictuseptin-1, identified in Cruziohyla calcarifer and Boana picturata respectively, has been carried out. The studies on Cruzioseptin-4 show that it is a cationic peptide with a chain of 23 amino acids that possess 52.17% of hydrophobic amino acids and a charge of + 1.2 at pH 7. Similarly, Pictuseptin-1 is a 22 amino acids peptide with a charge of + 3 at pH 7 and 45.45% of hydrophobic amino acids. Furthermore, the predominant secondary structure for both peptides is alpha-helical. The physicochemical properties were predicted using PepCalc and Bio-Synthesis; secondary structures using Jpred4 and PredictProtein; while molecular docking was performed using Autodock Vina. Geometry optimization of the peptides was done using the ONIOM hybrid method with the HF/6-31G basis set implemented in the Gaussian 09 program. Finally, the molecular docking study indicates that the viable mechanism of action for both peptides is through a targeted attack on the cell membrane of pathogens via electrostatic interactions with different membrane components, leading to cell lysis.

Optimization of sequence and chiral content enhances therapeutic potential of tilapia piscidin peptides

Because antimicrobial peptides (AMPs) often exhibit broad-spectrum bactericidal potency, we sought to develop peptide-based antimicrobials for potential clinical use against drug-resistant pathogens. To accomplish this goal, we first optimized the amino acid sequence of a broad-spectrum AMP known as Tilapia Piscidin 4 (TP4). Then, we used the optimized sequence to create a pair of heterochiral variants (TP4-α and TP4-β) with different percentages of D-enantiomers, as poly-L peptides often exhibit poor pharmacokinetic profiles. The conformations of the peptide pair exhibited inverted chirality according to CD and NMR spectroscopic analyses. Both heterochiral peptides displayed enhanced stability and low hemolysis activities. Irrespective of their different d-enantiomer contents, both heterochiral peptides exhibited bactericidal activities in the presence of human serum or physiological enzymes. However, the peptide with higher d-amino acid content (TP4-β) caused better bacterial clearance when tested in mice infected with NDM-1 K. pneumoniae. In addition, we observed a relatively higher hydrogen bonding affinity in a simulation of the interaction between TP4-β and a model bacterial membrane. In sum, our results demonstrate that the current design strategy may be applicable for development of new molecules with enhanced stability and in vivo antimicrobial activity.

Expression and purification of epinecidin-1 variant (Ac-Var-1) by acid cleavage

The demand for massive quantities of therapeutic active antimicrobial peptides (AMPs) is high due to their potential as alternatives to antibiotics. However, each antimicrobial peptide has unique properties, necessitating distinct synthesis and purification strategies for their large-scale production. In this study, we bio-synthesized and purified a functional enhanced variant of the AMP epinecidin-1, known as Ac-Var-1 (acid-cleavable variant-1). To generate the active peptide, we cloned the gene for Ac-Var-1 with acid-cleavable site (aspartic acid-proline) into the pET-32a expression vector, purified the fusion protein by His tag enrichment chromatography, and performed acid cleavage to release the active Ac-Var-1 peptide. After acid cleavage, the active Ac-Var-1 was purified and characterized by SDS-PAGE and mass spectrometry. The results from both techniques provided confirmation of the intactness of the purified Ac-Var-1. The Ac-Var-1 inhibited the growth of pathogenic Escherichia coli and Staphylococcus aureus. KEY POINTS : • Epinecidin-1 is a well-known antimicrobial peptide having multipotential bioactivities. • Epinecidin-1 variant is developed via the site-directed mutagenesis method to improve its structural stability and bioactivity. • AC-Var-1 development is an economical and easy method to remove peptide from tag protein.

Biochemistry, Mechanistic Intricacies, and Therapeutic Potential of Antimicrobial Peptides: An Alternative to Traditional Antibiotics

The emergence of drug-resistant strains of pathogens becomes a major obstacle to treating human diseases. Antibiotics and antivirals are in the application for a long time but now these drugs are not much effective anymore against disease-causing drugresistant microbes and gradually it is becoming a serious complication worldwide. The development of new antibiotics cannot be a stable solution to treat drug-resistant strains due to their evolving nature and escaping antibiotics. At this stage, antimicrobial peptides (AMPs) may provide us with novel therapeutic leads against drug-resistant pathogens. Structurally, antimicrobial peptides are mostly α-helical peptide molecules with amphiphilic properties that carry the positive charge (cationic) and belong to host defense peptides. These positively charged AMPs can interact with negatively charged bacterial cell membranes and may cause the alteration in electrochemical potential on bacterial cell membranes and consequently lead to the death of microbial cells. In the present study, we will elaborate on the implication of AMPs in the treatment of various diseases along with their specific structural and functional properties. This review will provide information which assists in the development of new synthetic peptide analogues to natural AMPs. These analogues will eliminate the limitations of natural AMPs like toxicity and severe hemolytic activities.

Sequential rearrangement and stereochemical reorganization to design an antimicrobial peptide with enhanced stability

Antimicrobial peptides (AMPs) are natural molecules that function within the innate immune system to counteract pathogenic invasion and minimize the detrimental consequences of infection. However, utilizing these molecules for medical applications has been challenging. In this study, we selected a model AMP with poor stability, Tilapia Piscidin 4 (TP4), and modified its sequence and chirality (TP4-γ) to improve its potential for clinical application. The strategy of chirality inversion was inspired by the cereulide peptide, which has a DDLL enantiomer pattern and exhibits exceptional stability. Sequential substitution of key residues and selective chirality inversion yielded a less toxic peptide with enhanced stability and notable antimicrobial activity. In addition to its superior stability profile and antimicrobial activity, TP4-γ treatment reduced the level of LPS-induced nitric oxide (NO) release in a macrophage cell line. This reduction in NO release may reflect anti-inflammatory properties, as NO is widely known to promote inflammatory processes. Hence, our heterochiral peptide construct shows a more suitable pharmacokinetic profile than its parental compound, and further studies are warranted to develop the molecule for potential clinical application.

Progressive Design of a Ranatuerin-2 Peptide from Amolops wuyiensis: Enhancement of Bioactivity and In Vivo Efficacy

Antimicrobial peptides (AMPs) that exert multiple functions are considered promising candidates to combat the bacterial drug resistance crisis. Nowadays, targeted peptide modification has been widely recognised to improve biological activity and make up for deficiencies in clinical applications such as toxicity. In this study, a helix-loop peptide was isolated and identified from the skin secretion of the Wuyi torrent frog Amolops wuyiensis, namely, ranatuerin-2-AW (R2AW) (GFMDTAKNVAKNVAATLLDKLKCKITGGC). Target modifications were made to R2AW to study the structure-activity relationships and to optimise its bioactivities. Five analogues were progressively designed via residue substitution and truncation and the antibacterial and anticancer activities were evaluated. We found that the serine-substitution and cyclic-domain-deletion products showed similar antibacterial activity to the natural peptide R2AW, implying that the disulphide bridge and Rana box were dispensable for the antibacterial activity of ranatuerin-2 peptides. Notably, the cationicity- and hydrophobicity-enhanced variant, [Lys4,19, Leu20]R2AW(1-22)-NH2, exhibited significantly optimised antibacterial and anticancer activities. Additionally, it killed bacteria by membrane disruption at a highly efficient rate. Moreover, [Lys4,19, Leu20]R2AW(1-22)-NH2 exerted potential in vivo efficacy in a methicillin-resistant Staphylococcus aureus (MRSA)-infected waxworm model. Overall, this study demonstrated some rational design ideas for optimising the dual antibacterial and anticancer activities of ranatuerin-2 peptides and it proposes [Lys4,19, Leu20]R2AW(1-22)-NH2 as an appealing candidate for therapeutic development.

Antimicrobial Peptides as Immunomodulators and Antimycobacterial Agents to Combat Mycobacterium tuberculosis: a Critical Review

Tuberculosis (TB) is a devastating disease foisting a significantly high morbidity, prepotent in low- and middle-income developing countries. Evolution of drug resistance among Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, has made the TB treatment more complicated. The protracted nature of present TB treatment, persistent and tolerant Mtb populations, interaction with antiretroviral therapy and existing toxicity concerned with conventional anti-TB drugs are the four major challenges inflicted with emergence of drug-resistant mycobacterial strains, and the standard medications are unable to combat these strains. These factors emphasize an exigency to develop new drugs to overcome these barriers in current TB therapy. With this regard, antimycobacterial peptides derived from various sources such as human cells, bacterial sources, mycobacteriophages, fungal, plant and animal sources could be considered as antituberculosis leads as most of these peptides are associated with dual advantages of having both bactericidal activity towards Mtb as well as immuno-regulatory property. Some of the peptides possess the additional advantage of interacting synergistically with antituberculosis medications too, thereby increasing their efficiency, underscoring the vigour of antimicrobial peptides (AMPs) as best possible alternative therapeutic candidates or adjuvants in TB treatment. Albeit the beneficiary features of these peptides, few obstacles allied with them like cytotoxicity and proteolytic degradation are matter of concerns too. In this review, we have focused on structural hallmarks, targeting mechanisms and specific structural aspects contributing to antimycobacterial activity and discovered natural and synthetic antimycobacterial peptides along with their sources, anti-TB, immuno-regulatory properties, merits and demerits and possible delivery methods of AMPs.

Deciphering Solution and Gas-Phase Interactions between Peptides and Lipids by Native Mass Spectrometry

Many biological processes depend on the interactions between proteins and lipids. Accordingly, the analysis of protein-lipid complexes has become increasingly important. Native mass spectrometry is often used to identify and characterize specific protein-lipid interactions. However, it requires the transfer of the analytes into the gas phase, where electrostatic interactions are enhanced and hydrophobic interactions do not exist. Accordingly, the question remains whether interactions that are observed in the gas phase accurately reflect interactions that are formed in solution. Here, we systematically explore noncovalent interactions between the antimicrobial peptide LL-37 and glycerophospholipids containing different headgroups or varying in fatty acyl chain length. We observe differences in peak intensities for different peptide-lipid complexes, as well as their relative binding strength in the gas phase. Accordingly, we found that ion intensities and gas-phase stability correlate well for complexes formed by electrostatic interactions. Probing hydrophobic interactions by varying the length of fatty acyl chains, we detected differences in ion intensities based on hydrophobic interactions formed in solution. The relative binding strength of these peptide-lipid complexes revealed only minor differences originating from van der Waals interactions and different binding modes of lipid headgroups in solution. In summary, our results demonstrate that hydrophobic interactions are reflected by ion intensities, while electrostatic interactions, including van der Waals interactions, determine the gas-phase stability of complexes.

Application Value of Antimicrobial Peptides in Gastrointestinal Tumors

Gastrointestinal cancer is a common clinical malignant tumor disease that seriously endangers human health and lacks effective treatment methods. As part of the innate immune defense of many organisms, antimicrobial peptides not only have broad-spectrum antibacterial activity but also can specifically kill tumor cells. The positive charge of antimicrobial peptides under neutral conditions determines their high selectivity to tumor cells. In addition, antimicrobial peptides also have unique anticancer mechanisms, such as inducing apoptosis, autophagy, cell cycle arrest, membrane destruction, and inhibition of metastasis, which highlights the low drug resistance and high specificity of antimicrobial peptides. In this review, we summarize the related studies on antimicrobial peptides in the treatment of digestive tract tumors, mainly oral cancer, esophageal cancer, gastric cancer, liver cancer, pancreatic cancer, and colorectal cancer. This paper describes the therapeutic advantages of antimicrobial peptides due to their unique anticancer mechanisms. The length, net charge, and secondary structure of antimicrobial peptides can be modified by design or modification to further enhance their anticancer effects. In summary, as an emerging cancer treatment drug, antimicrobial peptides need to be further studied to realize their application in gastrointestinal cancer diseases.

In silico molecular and functional characterization of a dual function antimicrobial peptide, hepcidin (GIFT-Hep), isolated from genetically improved farmed tilapia (GIFT, Oreochromis niloticus)

BACKGROUND

Antimicrobial peptides (AMPs), innate immune response molecules in organisms, are also known for their dual functionality, exemplified by hepcidin-an immunomodulator and iron regulator. Identifying and studying various AMPs from fish species can provide valuable insights into the immune profiles of aquaculturally significant fish, which can be made use of in its culture.

RESULTS

Hepcidin, a dual-function antimicrobial peptide, was isolated from the gill tissue of Genetically Improved Farmed Tilapia (GIFT-Hep). GIFT-Hep consists of a 90 amino acid pre-propeptide with a 24-mer signal, a 40-mer propeptide, and a 26-mer mature peptide region. The mature peptide had a molecular weight of 3015.61 Da, a theoretical pI of 8.78, a net charge of +4.25, and a protein-binding potential of 2.06 kcal/mol. Four disulfide bonds were formed by eight cysteine residues in the mature region. The presence of positively charged arginine residues renders the peptide 50% hydrophobic. Molecular analysis of GIFT-Hep revealed the presence of a furin propeptide convertase motif, RX(K/R)R, which facilitates trimming of the peptide to yield the mature GIFT-Hep. The hypothetical iron regulatory sequence, QSHLSL, was also identified in the mature peptide. In silico predictions about the characteristics of GIFT-Hep, such as charge, hydrophobicity, high surface accessibility, transmembrane helical regions, hydrophobic faces, hot spots, and cell-penetrating properties, suggest that the peptide functions as an iron regulatory antimicrobial agent.

CONCLUSIONS

This study reports a hepcidin antimicrobial peptide with both HAMP1 and HAMP2 properties isolated from genetically improved farmed tilapia, and further evaluation of the properties will prove the feasibility of GIFT-Hep being used as a therapeutant in aquaculture.

Aquiluscidin, a Cathelicidin from Crotalus aquilus, and the Vcn-23 Derivative Peptide, Have Anti-Microbial Activity against Gram-Negative and Gram-Positive Bacteria

Anti-microbial peptides play a vital role in the defense mechanisms of various organisms performing functions that range from the elimination of microorganisms, through diverse mechanisms, to the modulation of the immune response, providing protection to the host. Among these peptides, cathelicidins, a well-studied family of anti-microbial peptides, are found in various animal species, including reptiles. Due to the rise in anti-microbial resistance, these compounds have been suggested as potential candidates for developing new drugs. In this study, we identified and characterized a cathelicidin-like peptide called Aquiluscidin (Aq-CATH) from transcripts obtained from the skin and oral mucosa of the Querétaro's dark rattlesnake, Crotalus aquilus. The cDNA was cloned, sequenced, and yielded a 566-base-pair sequence. Using bioinformatics, we predicted that the peptide precursor contains a signal peptide, a 101-amino-acid conserved cathelin domain, an anionic region, and a 34-amino-acid mature peptide in the C-terminal region. Aq-CATH and a derived 23-amino-acid peptide (Vcn-23) were synthesized, and their anti-microbial activity was evaluated against various species of bacteria in in vitro assays. The minimal inhibitory concentrations against bacteria ranged from 2 to 8 μg/mL for both peptides. Furthermore, at concentrations of up to 50 μM, they exhibited no significant hemolytic activity (<2.3% and <1.2% for Aquiluscidin and Vcn-23, respectively) against rat erythrocytes and displayed no significant cytotoxic activity at low concentrations (>65% cell viability at 25 µM). Finally, this study represents the first identification of an antimicrobial peptide in Crotalus aquilus, which belongs to the cathelicidin family and exhibits the characteristic features of these peptides. Both Aq-CATH and its derived molecule, Vcn-23, displayed remarkable inhibitory activity against all tested bacteria, highlighting their potential as promising candidates for further antimicrobial research.

Benzyl stapled modification and anticancer activity of antimicrobial peptide A4K14-Citropin 1.1

A4K14-Citropin 1.1 (GLFAVIKKVASVIKGL-NH2) is a derived antimicrobial peptide (AMP) with a more stable α-helical structure at the C-terminal compared to prototype Citropin 1.1 which was obtained from glandular skin secretions of Australian freetail lizards. In a previous report, A4K14-Citropin 1.1 has been considered as an anti-cancer lead compound. However, linear peptides are difficult to maintain stable secondary structure, resulted in poor pharmacokinetic properties. In this study, we designed and synthesized a series of benzyl-stapled derivatives of A4K14-Citropin 1.1. And their physical and chemical properties, as well as biological activity, were both explored. The result showed that AC-CCSP-2-o and AC-CCSP-3-o exhibited a higher degree of helicity and greater anti-cancer activity compared with the prototype peptide. Besides, there was no significant difference in the hemolytic effect between the stapled peptides and the prototype peptide. AC-CCSP-2-o and AC-CCSP-3-o could serve as promising anti-cancer lead compounds for the novel anti-cancer drug development.

Transcriptome analysis of Corvus splendens reveals a repertoire of antimicrobial peptides

Multidrug resistance has become a global health problem associated with high morbidity and mortality. Antimicrobial peptides have been acknowledged as potential leads for prospective anti-infectives. Owing to their scavenging lifestyle, Corvus splendens is thought to have developed robust immunity to pathogens found in their diet, implying that they have evolved mechanisms to resist infection. In the current study, the transcriptome of C. splendens was sequenced, and de novo assembled to identify the presence of antimicrobial peptide genes. 72.09 million high-quality clean reads were obtained which were then de novo assembled into 3,43,503 transcripts and 74,958 unigenes. About 37,559 unigenes were successfully annotated using SwissProt, Pfam, GO, and KEGG databases. A search against APD3, CAMPR3 and LAMP databases identified 63 AMP candidates belonging to more than 20 diverse families and functional classes. mRNA of AvBD-2, AvBD-13 and CATH-2 were found to be differentially expressed between the three tested crows as well as among the tissues. We also characterized Corvus Cathelicidin 2 (CATH-2) to gain knowledge of its antimicrobial mechanisms. The CD spectroscopy of synthesized mature Corvus CATH-2 peptide displayed an amphipathic α-helical structure. Though the synthetic CATH-2 caused hemolysis of human RBC, it also exhibited antimicrobial activity against E. coli, S. aureus, and B. cereus. Docking simulation results revealed that this peptide could bind to the LPS binding site of MD-2, which may prevent LPS from entering the MD-2 binding pocket, and trigger TLR4 signaling pathway. The Corvus CATH-2 characterized in this study could aid in the development of novel therapeutics.

Broad-Spectrum Antimicrobial Activity of Synthetic Peptides GV185 and GV187

Optimizing synthetic antimicrobial peptides for safe and enhanced activity against fungal and bacterial pathogens is useful for genetic engineering of plants for resistance to plant pathogens and their associated mycotoxins. Nine synthetic peptides modeled after lytic peptides tachyplesin 1, D4E1 from cecropin A, and protegrin 1 were added to germinated spores of fungal species Aspergillus flavus, Rhizopus stolonifer, Fusarium oxysporum f. sp. vasinfectum, F. verticillioides, F. graminearum, Claviceps purpurea, Verticillium dahliae, and Thielaviopsis basicola and bacterial cultures of Pseudomonas syringae pv. tabaci and Xanthomonas campestris pv. campestris at different doses and inhibitory dose response curves, and were modeled to assess antimicrobial activity. Peptides GV185 and GV187, modified from tachyplesin 1, had superior abilities to inhibit fungal and bacterial growth (50% inhibitory concentrations [IC50] ranging from 0.1 to 8.7 µM). R. stolonifer (IC50 = 8.1 µM), A. flavus (IC50 = 3.1 µM), and F. graminearum (IC50 = 2.2 µM) were less inhibited by GV185 and GV187 than all the remaining fungi (IC50 = 1.4 µM) and bacteria (IC50 = 0.1 µM). Of the remaining peptides, GV193, GV195, and GV196 (IC50 range of 0.9 to 6.6 µM) inhibited fungal growth of A. flavus, F. verticillioides, and F. graminearum less than GV185 and GV187 (IC50 range of 0.8 to 3.9 µM), followed by GV197 (IC50 range of 0.8 to 9.1 µM), whereas GV190 and GV192 inhibited poorly (IC50 range of 28.2 to 36.6 µM and 15.5 to 19.4 µM, respectively) and GV198 stimulated growth. GV185 and GV187 had slightly weaker hydrophobic and cationic residues than other tachyplesin 1 modified peptides but still had unexpectedly high lytic activity. Germinated fungal spores of R. stolonifer and F. graminearum exposed to these two peptides and D4E1 and AGM182 appeared wrinkled, with perforations near potential cytoplasmic leakage, which provided evidence of plasma membrane and cell wall lysis. We conclude that peptides GV185 and GV187 are promising candidates for genetic engineering of crops for resistance to plant-pathogenic bacteria and fungi, including A. flavus and aflatoxin contamination.

The Contribution of Antimicrobial Peptides to Immune Cell Function: A Review of Recent Advances

The development of novel antimicrobial agents to replace antibiotics has become urgent due to the emergence of multidrug-resistant microorganisms. Antimicrobial peptides (AMPs), widely distributed in all kingdoms of life, present strong antimicrobial activity against a variety of bacteria, fungi, parasites, and viruses. The potential of AMPs as new alternatives to antibiotics has gradually attracted considerable interest. In addition, AMPs exhibit strong anticancer potential as well as anti-inflammatory and immunomodulatory activity. Many studies have provided evidence that AMPs can recruit and activate immune cells, controlling inflammation. This review highlights the scientific literature focusing on evidence for the anti-inflammatory mechanisms of different AMPs in immune cells, including macrophages, monocytes, lymphocytes, mast cells, dendritic cells, neutrophils, and eosinophils. A variety of immunomodulatory characteristics, including the abilities to activate and differentiate immune cells, change the content and expression of inflammatory mediators, and regulate specific cellular functions and inflammation-related signaling pathways, are summarized and discussed in detail. This comprehensive review contributes to a better understanding of the role of AMPs in the regulation of the immune system and provides a reference for the use of AMPs as novel anti-inflammatory drugs for the treatment of various inflammatory diseases.

Bio-inspired peptide-conjugated liposomes for enhanced planktonic bacteria killing and biofilm eradication

Developing new antimicrobial agents has become an urgent task to address the increasing prevalence of multidrug-resistant pathogens and the emergence of biofilms. Cationic antimicrobial peptides (AMPs) have been regarded as promising candidates due to their unique non-specific membrane rupture mechanism. However, a series of problems with the peptides hindered their practical application due to their high toxicity and low bioactivity and stability. Here, inspired by broadening the application of cell-penetrating peptides (CPPs), we selected five different sequences of cationic peptides which are considered as both CPPs and AMPs, and developed a biomimetic strategy to construct cationic peptide-conjugated liposomes with the virus-like structure for both enhancements of antibacterial efficacy and biosafety. The correlation between available peptide density/peptide variety and antimicrobial capabilities was evaluated from quantitative perspectives. Computational simulation and experimental investigations assisted to identify the optimal peptide-conjugated liposomes and revealed that the designed system provides high charge density for enhanced anionic bacterial membrane binding capability without compromised cytotoxicity, being capable of enhanced antibacterial efficacy of bacteria/biofilm of clinically important pathogens. The bio-inspired design has shown enhanced therapeutic efficiency of peptides and may promote the development of next-generation antimicrobials.

Antimicrobial Peptides in Infectious Diseases and Beyond-A Narrative Review

Despite recent medical research and clinical practice developments, the development of antimicrobial resistance (AMR) significantly limits therapeutics for infectious diseases. Thus, novel treatments for infectious diseases, especially in this era of increasing AMR, are urgently needed. There is ongoing research on non-classical therapies for infectious diseases utilizing alternative antimicrobial mechanisms to fight pathogens, such as bacteriophages or antimicrobial peptides (AMPs). AMPs are evolutionarily conserved molecules naturally produced by several organisms, such as plants, insects, marine organisms, and mammals, aiming to protect the host by fighting pathogenic microorganisms. There is ongoing research regarding developing AMPs for clinical use in infectious diseases. Moreover, AMPs have several other non-medical applications in the food industry, such as preservatives, animal husbandry, plant protection, and aquaculture. This review focuses on AMPs, their origins, biology, structure, mechanisms of action, non-medical applications, and clinical applications in infectious diseases.

Insect Meals and Insect Antimicrobial Peptides as an Alternative for Antibiotics and Growth Promoters in Livestock Production

The extensive use of antibiotics in animal production has led to the development of antibiotic-resistant microorganisms and the search for alternative antimicrobial agents in animal production. One such compound may be antimicrobial peptides (AMPs), which are characterized by, among others, a wide range of biocidal activity. According to scientific data, insects produce the largest number of antimicrobial peptides, and the changing EU legislation has allowed processed animal protein derived from insects to be used in feed for farm animals, which, in addition to a protein supplement, may prove to be an alternative to antibiotics and antibiotic growth promoters due to their documented beneficial impact on livestock health. In animals that were fed feeds with the addition of insect meals, changes in their intestinal microbiota, strengthened immunity, and increased antibacterial activity were confirmed to be positive effects obtained thanks to the insect diet. This paper reviews the literature on sources of antibacterial peptides and the mechanism of action of these compounds, with particular emphasis on insect antibacterial peptides and their potential impact on animal health, and legal regulations related to the use of insect meals in animal nutrition.