Combination Nano-Delivery Systems Remodel the Immunosuppressive Tumor Microenvironment for Metastatic Triple-Negative Breast Cancer Therapy

Triple-negative breast cancer (TNBC) is an aggressive type of breast cancer for which effective therapies are lacking. Targeted remodeling of the immunosuppressive tumor microenvironment (TME) and activation of the body's immune system to fight tumors with well-designed nanoparticles have emerged as pivotal breakthroughs in tumor treatment. To simultaneously remodel the immunosuppressive TME and trigger immune responses, we designed two potential therapeutic nanodelivery systems to inhibit TNBC. First, the bromodomain-containing protein 4 (BRD4) inhibitor JQ1 and the cyclooxygenase-2 (COX-2) inhibitor celecoxib (CXB) were coloaded into chondroitin sulfate (CS) to obtain CS@JQ1/CXB nanoparticles (NPs). Then, the biomimetic nanosystem MM@P3 was prepared by coating branched polymer poly(β-amino ester) self-assembled NPs with melittin embedded macrophage membranes (MM). Both in vitro and in vivo, the CS@JQ1/CXB and MM@P3 NPs showed excellent immune activation efficiencies. Combination treatment exhibited synergistic cytotoxicity, antimigration ability, and apoptosis-inducing and immune activation effects on TNBC cells and effectively suppressed tumor growth and metastasis in TNBC tumor-bearing mice by activating the tumor immune response and inhibiting angiogenesis. In summary, this study offers a novel combinatorial immunotherapeutic strategy for the clinical TNBC treatment.

Immunomodulatory activities and biomedical applications of melittin and its recent advances

Melittin (MLT), a peptide containing 26 amino acids, is a key constituent of bee venom. It comprises ∼40%-60% of the venom's dry weight and is the main pricing index for bee venom, being the causative factor of pain. The unique properties of MLT extracted from bee venom have made it a very valuable active ingredient in the pharmaceutical industry as this cationic and amphipathic peptide has propitious effects on human health in diverse biological processes. It has the ability to strongly impact the membranes of cells and display hemolytic activity with anticancer characteristics. However, the clinical application of MLT has been limited by its severe hemolytic activity, which poses a challenge for therapeutic use. By employing more efficient mechanisms, such as modifying the MLT sequence, genetic engineering, and nano-delivery systems, it is anticipated that the limitations posed by MLT can be overcome, thereby enabling its wider application in therapeutic contexts. This review has outlined recent advancements in MLT's nano-delivery systems and genetically engineered cells expressing MLT and provided an overview of where the MLTMLT's platforms are and where they will go in the future with the challenges ahead. The focus is on exploring how these approaches can overcome the limitations associated with MLT's hemolytic activity and improve its selectivity and efficacy in targeting cancer cells. These advancements hold promise for the creation of innovative and enhanced therapeutic approaches based on MLT for the treatment of cancer.

The Synthetic Antimicrobial Peptide Derived From Melittin Displays Low Toxicity and Anti-infectious Properties

The low stability and nonspecific toxicity are the main limiting factors for the clinical applications of melittin (MLT). This study aimed to design and synthesize new analogs of MLT to increase stability, reduce toxicity, and retain their antimicrobial properties against bacterial pathogens. At first, peptide analogs were designed computationally by inducing single mutations in MLT peptides and evaluating their physicochemical properties. The stability of the analogs with the highest scores was determined by Gromacs software. In vitro assays were performed to examine the antimicrobial activity and toxicity of the selected analogs. Two peptide analogs, M1 and M2, were selected based on the SVM score in cell PPD. The M1 analog was created by replacing alanine with leucine on the 15th. The M2 analog was designed by substituting alanine with leucine and isoleucine with arginine at the 15th and 17th positions. According to the Gromacs results, the M2 peptide indicated more stability. RMSD and RMSF results showed no undesirable fluctuations during the 200 ns MD simulation. The MIC and MBC values for the M1 peptide were calculated in a range of 8-128 μg/ml, while the M2 peptide limited the bacterial growth to 32-128 μg/mL. Both peptides indicated less toxicity than natural MLT, based on MTT assay results. The hemolytic activity of the M1 analog was more than M2 at 16 μg/mL concentration. M1 peptide displayed the highest selectivity index against S. aureus and A. baumannii, which were approximately 5.27-fold improvements compared to MLT. In conclusion, we introduced two analogs of MLT with low toxicity, low hemolytic activity, and higher stability, along with retaining antimicrobial properties against gram-negative and positive bacteria.

Standardized guidelines for Africanized honeybee venom production needed for development of new apilic antivenom

Africanized bees have spread across the Americas since 1956 and consequently resulted in human and animal deaths attributed to massive attacks related to exposure from Argentina to the USA. In Brazil, more than 100,000 accidents were registered in the last 5 years with a total of 303 deaths. To treat such massive attacks, Brazilian researchers developed the first specific antivenom against Africanized honey bee sting exposure. This unique product, the first of its kind in the world, has been safely tested in 20 patients during a Phase 2 clinical trial. To develop the antivenom, a standardized process was undertaken to extract primary venom antigens from the Africanized bees for immunization of serum-producing horses. This process involved extracting, purifying, fractionating, characterizing, and identifying the venom (apitoxin) employing mass spectrometry to generate standardized antigen for hyperimmunization of horses using the major toxins (melittin and its isoforms and phospholipase A2). The current guide describes standardization of the entire production chain of venom antigens in compliance with good manufacturing practices (GMP) required by regulatory agencies. Emphasis is placed upon the welfare of bees and horses during this process, as well as the development of a new biopharmaceutical to ultimately save lives.

How do Antimicrobial Peptides Interact with the Outer Membrane of Gram-Negative Bacteria? Role of Lipopolysaccharides in Peptide Binding, Anchoring, and Penetration

Gram-negative bacteria possess a complex structural cell envelope that constitutes a barrier for antimicrobial peptides that neutralize the microbes by disrupting their cell membranes. Computational and experimental approaches were used to study a model outer membrane interaction with an antimicrobial peptide, melittin. The investigated membrane included di[3-deoxy-d-manno-octulosonyl]-lipid A (KLA) in the outer leaflet and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) in the inner leaflet. Molecular dynamics simulations revealed that the positively charged helical C-terminus of melittin anchors rapidly into the hydrophilic headgroup region of KLA, while the flexible N-terminus makes contacts with the phosphate groups of KLA, supporting melittin penetration into the boundary between the hydrophilic and hydrophobic regions of the lipids. Electrochemical techniques confirmed the binding of melittin to the model membrane. To probe the peptide conformation and orientation during interaction with the membrane, polarization modulation infrared reflection absorption spectroscopy was used. The measurements revealed conformational changes in the peptide, accompanied by reorientation and translocation of the peptide at the membrane surface. The study suggests that melittin insertion into the outer membrane affects its permeability and capacitance but does not disturb the membrane's bilayer structure, indicating a distinct mechanism of the peptide action on the outer membrane of Gram-negative bacteria.

Upgrading of the general AMBER force field 2 for fluorinated alcohol biosolvents: A validation for water solutions and melittin solvation

The standard GAFF2 force field parameterization has been refined for the fluorinated alcohols 2,2,2-trifluoroethanol (TFE), 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), and 1,1,1,3,3,3-hexafluoropropan-2-one (HFA), which are commonly used to study proteins and peptides in biomimetic media. The structural and dynamic properties of both proteins and peptides are significantly influenced by the biomimetic environment created by the presence of these cosolvents in aqueous solutions. Quantum mechanical calculations on stable conformers were used to parameterize the atomic charges. Different systems, such as pure liquids, aqueous solutions, and systems formed by melittin protein and cosolvent/water solutions, have been used to validate the new models. The calculated macroscopic and structural properties are in agreement with experimental findings, supporting the validity of the newly proposed models.

The current landscape of the antimicrobial peptide melittin and its therapeutic potential

Melittin, a main component of bee venom, is a cationic amphiphilic peptide with a linear α-helix structure. It has been reported that melittin can exert pharmacological effects, such as antitumor, antiviral and anti-inflammatory effects in vitro and in vivo. In particular, melittin may be beneficial for the treatment of diseases for which no specific clinical therapeutic agents exist. Melittin can effectively enhance the therapeutic properties of some first-line drugs. Elucidating the mechanism underlying melittin-mediated biological function can provide valuable insights for the application of melittin in disease intervention. However, in melittin, the positively charged amino acids enables it to directly punching holes in cell membranes. The hemolysis in red cells and the cytotoxicity triggered by melittin limit its applications. Melittin-based nanomodification, immuno-conjugation, structural regulation and gene technology strategies have been demonstrated to enhance the specificity, reduce the cytotoxicity and limit the off-target cytolysis of melittin, which suggests the potential of melittin to be used clinically. This article summarizes research progress on antiviral, antitumor and anti-inflammatory properties of melittin, and discusses the strategies of melittin-modification for its future potential clinical applications in preventing drug resistance, enhancing the selectivity to target cells and alleviating cytotoxic effects to normal cells.

Glutamate affects self-assembly, protein corona, and anti-4 T1 tumor effects of melittin/vitamin E-succinic acid-(glutamate)n nanoparticles

Melittin (M) has attracted increasing attention for its significant antitumor effects and various immunomodulatory effects. However, various obstacles such as the short plasma half-life and adverse reactions restrict its application. This study aimed to systematically investigate the self-assembly mechanism, components of the protein corona, targeting behavior, and anti-4 T1 tumor effect of vitamin E-succinic acid-(glutamate)n /melittin nanoparticles with varying amounts of glutamic acid. Here, we present a new vitamin E-succinic acid-(glutamate)5 (E5), vitamin E-succinic acid-(glutamate)10 (E10) or vitamin E-succinic acid-(glutamate)15 (E15), and their co-assembly system with positively charged melittin in water. The molecular dynamics simulations demonstrated that the electrostatic energy and van der Waals force in the system decreased significantly with the increase in the amount of glutamic acid. The melittin and E15 system exhibited the optimal stability for nanoparticle self-assembly. When nanoparticles derived from different self-assembly systems were co-incubated with plasma from patients with breast cancer, the protein corona showed heterogeneity. In vivo imaging demonstrated that an increase in the number of glutamic acid residues enhanced circulation duration and tumor-targeting effects. Both in vitro and in vivo antitumor evaluation indicated a significant increase in the antitumor effect with the addition of glutamic acid. According to our research findings, the number of glutamic acid residues plays a crucial role in the targeted delivery of melittin for immunomodulation and inhibition of 4 T1 breast cancer. Due to the self-assembly capabilities of vitamin E-succinic acid-(glutamate)n in water, these nanoparticles carry significant potential for delivering cationic peptides such as melittin.

Melittin: a possible regulator of cancer proliferation in preclinical cell culture and animal models

BACKGROUND

Melittin is a water-soluble cationic peptide derived from bee venom that has been thoroughly studied for the cure of different cancers. However, the unwanted interactions of melittin produce hemolytic and cytotoxic effects that hinder their therapeutic applications. To overcome the shortcomings, numerous research groups have adopted different approaches, including conjugation with tumor-targeting proteins, gene therapy, and encapsulation in nanoparticles, to reduce the non-specific cytotoxic effects and potentiate their anti-cancerous activity.

PURPOSE

This article aims to provide mechanistic insights into the chemopreventive activity of melittin and its nanoversion in combination with standard anti-cancer drugs for the treatment of cancer.

METHODS

We looked over the pertinent research on melittin's chemopreventive properties in online databases such as PubMed and Scopus.

CONCLUSION

In the present article, the anti-cancerous effects of melittin on different cancers have been discussed very nicely, as have their possible mechanisms of action to act against different tumors. Besides, it interacts with different signal molecules that regulate the diverse pathways of cancerous cells, such as cell cycle arrest, apoptosis, metastasis, angiogenesis, and inflammation. We also discussed the recent progress in the synergistic combination of melittin with standard anti-cancer drugs and a nano-formulated version of melittin for targeted delivery to improve its anticancer potential.

Recent advances in melittin-based nanoparticles for antitumor treatment: from mechanisms to targeted delivery strategies

As a naturally occurring cytolytic peptide, melittin (MLT) not only exhibits a potent direct tumor cell-killing effect but also possesses various immunomodulatory functions. MLT shows minimal chances for developing resistance and has been recognized as a promising broad-spectrum antitumor drug because of this unique dual mechanism of action. However, MLT still displays obvious toxic side effects during treatment, such as nonspecific cytolytic activity, hemolytic toxicity, coagulation disorders, and allergic reactions, seriously hampering its broad clinical applications. With thorough research on antitumor mechanisms and the rapid development of nanotechnology, significant effort has been devoted to shielding against toxicity and achieving tumor-directed drug delivery to improve the therapeutic efficacy of MLT. Herein, we mainly summarize the potential antitumor mechanisms of MLT and recent progress in the targeted delivery strategies for tumor therapy, such as passive targeting, active targeting and stimulus-responsive targeting. Additionally, we also highlight the prospects and challenges of realizing the full potential of MLT in the field of tumor therapy. By exploring the antitumor molecular mechanisms and delivery strategies of MLT, this comprehensive review may inspire new ideas for tumor multimechanism synergistic therapy.

[Identification of Melittin-Like Proteins with a Molecular Weight of 67 κDa that Interact with Na^(+)/K^(+)-ATPase]

Melittin, a peptide from bee venom, was found to be able to interact with many proteins, including calmodulin target proteins and ion-transporting P-type ATPases. It is assumed that melittin mimics a protein module involved in protein-protein interactions within cells. Previously, a Na^(+)/K^(+)-ATPase containing the α1 isoform of the catalytic subunit was found to co-precipitate with a protein with a molecular weight of about 70 κDa that interacts with antibodies against melittin by cross immunoprecipitation. In the presence of a specific Na^(+)/K^(+)-ATPase inhibitor (ouabain), the amount of protein with a molecular weight of 70 κDa interacting with Na^(+)/K^(+)-ATPase increases. In order to identify melittin-like protein from murine kidney homogenate, a fraction of melittin-like proteins with a molecular weight of approximately 70 κDa was obtained using affinity chromatography with immobilized antibodies specific to melittin. By mass spectrometry analysis, the obtained protein fraction was found to contain three molecular chaperones of Hsp70 superfamily: mitochondrial mtHsp70 (mortalin), Hsp73, Grp78 (BiP) of endoplasmic reticulum. These data suggest that chaperones from the HSP-70 superfamily contain a melittin-like module.

Non-Gaussian Diffusion of Individual Lipids Unveils the Unique Peptide-Membrane Interaction Dynamics

The dynamics of protein (or peptide)-membrane interactions plays a central role in cellular functions; however, the underlying mechanisms remain unclear. Herein, through analyzing the diffusion of individual lipids in a bilayer membrane during the membrane actions of typical peptides (e.g., pore-forming peptide melittin and cell-penetrating peptide TAT) at varying concentrations, the spatial heterogeneity as well as temporal dynamics of lipid motions were investigated which showed close correlation with the peptide action mechanism. Specifically, the spatial heterogeneity of lipid diffusion was characterized by the non-Gaussianity of lipid trajectories, which was further decomposed into two basic diffusion modes; moreover, the temporal evolution of the Gaussian fitting parameters provided quantitative information on the varying metastable interaction states between peptides and the membrane (e.g., peptide landing, membrane insertion, and equilibrium). Generally, this work gives an insight into the correlation between single-lipid diffusion and function-realization of membrane-active peptides.

The Pharmacological Potential of Novel Melittin Variants from the Honeybee and Solitary Bees against Inflammation and Cancer

The venom of honeybees is composed of numerous peptides and proteins and has been used for decades as an anti-inflammatory and anti-cancer agent in traditional medicine. However, the bioactivity of specific biomolecular components has been evaluated for the predominant constituent, melittin. So far, only a few melittin-like peptides from solitary bee species have been investigated, and the molecular mechanisms of bee venoms as therapeutic agents remain largely unknown. Here, the preclinical pharmacological activities of known and proteo-transcriptomically discovered new melittin variants from the honeybee and more ancestral variants from phylogenetically older solitary bees were explored in the context of cancer and inflammation. We studied the effects of melittin peptides on cytotoxicity, second messenger release, and inflammatory markers using primary human cells, non-cancer, and cancerous cell lines. Melittin and some of its variants showed cytotoxic effects, induced Ca²⁺ signaling and inhibited cAMP production, and prevented LPS-induced NO synthesis but did not affect the IP3 signaling and pro-inflammatory activation of endothelial cells. Compared to the originally-described melittin, some phylogenetically more ancestral variants from solitary bees offer potential therapeutic modalities in modulating the in vitro inflammatory processes, and hindering cancer cell viability/proliferation, including aggressive breast cancers, and are worth further investigation.

Organizations of melittin peptides after spontaneous penetration into cell membranes

The antimicrobial peptide, melittin, is a potential next-generation antibiotic because melittin can spontaneously form pores in bacterial cell membranes and cause cytoplasm leakage. However, the organizations of melittin peptides in cell membranes remain elusive, which impedes the understanding of the poration mechanism. In this work, we use coarse-grained and all-atom molecular dynamics (MD) simulations to investigate the organizations of melittin peptides during and after spontaneous penetration into DPPC/POPG lipid bilayers. We find that the peptides in lipid bilayers adopt either a transmembrane conformation or a U-shaped conformation, which are referred to as T- and U-peptides, respectively. Several U-peptides and/or T-peptides aggregate to form stable pores. We analyze a T-pore consisting of four T-peptides and a U-pore consisting of three U-peptides and one T-peptide. In both pores, peptides are organized in a manner such that polar residues face inward and hydrophobic residues face outward, which stabilizes the pores and produces water channels. Compared with the U-pore, the T-pore has lower energy, larger pore diameter, and higher permeability. However, the T-pore occurs less frequently than the U-pore in our simulations, probably because the formation of the T-pore is kinetically slower than the U-pore. The stability and permeability of both pores are confirmed by 300 ns all-atom MD simulations. The peptide organizations obtained in this work should deepen the understanding of the stability, poration mechanism, and permeability of melittin, and facilitate the optimization of melittin to enhance the antibacterial ability.

Short-Chained Alcohols Make Membrane Surfaces Conducive for Melittin Action: Implication for the Physiological Role of Alcohols in Cells

Alcohols are a part of cellular metabolism, but their physiological roles are not well understood. We investigated the effects of short-chain alcohols on Daphnia pulex and model membranes mimicking the lipid composition of eukaryotic inner mitochondrial membranes. We also studied the synergistic effects of alcohols with the bee venom membrane-active peptide, melittin, which is structurally similar to endogenous membrane-active peptides. The alcohols, from ethanol to octanol, gradually decreased the heart rate and the mitochondrial ATP synthesis of daphnia; in contrast, in combination with melittin, which exerted no sizeable effect, they gradually increased both the heart rate and the ATP synthesis. Lipid packing and the order parameter of oriented films, monitored by EPR spectroscopy of the spin-labeled probe 5-doxylstrearic acid, revealed gradual alcohol-assisted bilayer to non-bilayer transitions in the presence of melittin; further, while the alcohols decreased, in combination with melittin they increased the order parameter of the film, which is attributed to the alcohol-facilitated association of melittin with the membrane. A ¹H-NMR spectroscopy of the liposomes confirmed the enhanced induction of a non-bilayer lipid phase that formed around the melittin, without the permeabilization of the liposomal membrane. Our data suggest that short-chain alcohols, in combination with endogenous peptides, regulate protein functions via modulating the lipid polymorphism of membranes.

Studying the Mechanics of Membrane Permeabilization through Mechanoelectricity

In this research, real-time monitoring of lipid membrane disruption is made possible by exploiting the dynamic properties of model lipid bilayers formed at oil-water interfaces. This involves tracking an electrical signal generated through rhythmic membrane perturbation translated into the adsorption and penetration of charged species within the membrane. Importantly, this allows for the detection of membrane surface interactions that occur prior to pore formation that may be otherwise undetected. The requisite dynamic membranes for this approach are made possible through the droplet interface bilayer (DIB) technique. Membranes are formed at the interface of lipid monolayer-coated aqueous droplets submerged in oil. We present how cyclically alternating the membrane area leads to the generation of mechanoelectric current. This current is negligible without a transmembrane voltage until a composition mismatch between the membrane monolayers is produced, such as a one-sided accumulation of disruptive agents. The generated mechanoelectric current is then eliminated when an applied electric field compensates for this asymmetry, enabling measurement of the transmembrane potential offset. Tracking the compensating voltage with respect to time then reveals the gradual accumulation of disruptive agents prior to membrane permeabilization. The innovation of this work is emphasized in its ability to continuously track membrane surface activity, highlighting the initial interaction steps of membrane disruption. In this paper, we begin by validating our proposed approach against measurements taken for fixed composition membranes using standard electrophysiological techniques. Next, we investigate surfactant adsorption, including hexadecyltrimethylammonium bromide (CTAB, cationic) and sodium decyl sulfate (SDS, anionic), demonstrating the ability to track adsorption prior to disruption. Finally, we investigate the penetration of lipid membranes by melittin, confirming that the peptide insertion and disruption mechanics are, in part, modulated by membrane composition.

Stable Loading and Delivery of Melittin with Lipid-Coated Polymeric Nanoparticles for Effective Tumor Therapy with Negligible Systemic Toxicity

Melittin is a potential anticancer candidate with remarkable antitumor activity and ability to overcome tumor drug resistance. However, the clinical applications of melittin are largely restricted by its severe hemolytic activity and nonspecific cytotoxicity after systemic administration. Here, a biocompatible and stable melittin-loaded lipid-coated polymeric nanoparticle (MpG@LPN) formulation that contains a melittin/poly-γ-glutamic acid nanoparticle inner core, a lipid membrane middle layer, and a polyethylene glycol (PEG) and PEG-targeting molecule outer shell was designed. The formulations were prepared by applying a self-assembly procedure based on intermolecular interactions, including electrostatic attraction and hydrophobic effect. The core-shell MpG@LPN presented high efficiency for melittin encapsulation and high stability in physiological conditions. Hemolysis and cell proliferation assays showed that the PEG-modified MpG@LPN had almost no hemolytic activity and nonspecific cytotoxicity even at high concentrations. The modification of targeting molecules on the MpG@LPNs allowed for the selective binding with target tumor cells and cytolytic activity via apoptosis induction. In vivo experiments revealed that MpG@LPNs can remarkably inhibit the growth of tumors without the occurrence of hemolysis and tissue toxicity. Results suggested that the developed MpG@LPN with a core-shell structure can effectively address the main obstacles of melittin in clinical applications and has great potential in cancer treatment.

A Supramolecular Antidote to Macromolecular Toxins Prepared through Coassembly of Macrocyclic Amphiphiles

Poisoning is a leading cause of admission to medical emergency departments and intensive care units. Supramolecular detoxification, which involves injecting supramolecular receptors that bind with toxins to suppress their biological activity, is an emerging strategy for poisoning treatment; it has few requirements and a broad application scope. However, it is still a formidable challenge to design supramolecular therapeutic materials as an antidote to macromolecular toxins, because the large size, flexible conformation, and presence of multiple and diverse binding sites of biomacromolecules hinder their recognition. Herein, a supramolecular antidote to macromolecular toxins is developed through the coassembly of macrocyclic amphiphiles, relying on heteromultivalent recognition between the coassembled components and toxic macromolecules. The coassembly of amphiphilic cyclodextrin and calixarene strongly and selectively captures melittin, a toxin studied herein; this imparts various therapeutic effects such as inhibiting the interactions of melittin with cell membranes, alleviating melittin cytotoxicity and hemolytic toxicity, reducing the mortality rate of melittin-poisoned mice, and mitigating damage to major organs. The use of the proposed antidote overcomes the limitation of supramolecular detoxification applicability to only small-molecular toxins. The antidote can also detoxify other macromolecular toxins as long as selective and strong binding is achieved because of the coassembling tunability.

Effective inhibition of Clostridioides difficile by the novel peptide CM-A

Clostridioides difficile infection is the most common cause of nosocomial and antibiotic-associated diarrhea. C. difficile treatment is increasingly likely to fail, and the recurrence rate is high. Antimicrobial peptides are considered an alternative treatment for many infectious diseases, including those caused by antibiotic resistant bacteria. In the present study, we identified a CM peptide, a hybrid of cecropin A and melittin, and its derivative which possesses potent antimicrobial activity against C. difficile strain 630. CM peptide exhibited antibacterial activity with minimum inhibitory concentration of 3.906 μg/ml (2.21 μM). A modified derivative of CM, CM-A, exhibited even greater activity with a minimum inhibitory concentration of 1.953 μg/ml (1.06 μM) and a minimum bactericidal concentration of 7.8125 μg/ml (4.24 μM), which indicates that CM-A peptide is more efficient than its parent peptide. A fluorescence-activated cell sorter analysis revealed that the membrane of C. difficile 630 could be an important target for CM-A. This peptide induced high levels of cell depolarization and cell permeability on C. difficile cell membrane. Moreover, electron microscopy imaging showed that CM-A interferes with the C. difficile cell membrane. Hence, the antimicrobial peptide CM-A may represent a promising novel approach for the treatment of C. difficile infections.

Prevention the formation of biofilm on orthopedic implants by melittin thin layer on chitosan/bioactive glass/vancomycin coatings

Methicillin-resistant and Vancomycin-resistant Staphylococcus aureus bacteria (MRSA and VRSA, respectively) can seriously jeopardizes bone implants. This research aimed to examine the potential synergistic effects of Melittin and vancomycin in preventing MRSA and VRSA associated bone implant infections. Chitosan/bioactive glass nanoparticles/vancomycin composites were coated on hydrothermally etched titanium substrates by casting method. The composite coatings were coated by Melittin through drop casting technique. Melittin raised the proliferation of MC3T3 cells, making it an appropriate option as osteoinductive and antibacterial substance in coatings of orthopedic implants. Composite coatings having combined vancomycin and Melittin eliminated both planktonic and adherent MRSA and VRSA bacteria, whereas coatings containing one of them failed to kill the whole VRSA bacteria. Therefore, chitosan/bioactive glass/vancomycin/Melittin coating can be used as a bone implant coating because of its anti-infective properties.

Redox-Sensitive Nanocomplex for Targeted Delivery of Melittin

Although peptide therapeutics have been explored for decades, the successful delivery of potent peptides in vitro and in vivo remains challenging due to the poor stability, low cell permeability, and off-target effects. We developed a redox sensitive polymer-based nanocomplex which can efficiently and stably deliver the peptide drug melittin for cancer therapy. The nanocomplex selectively targets cancer cells through lactobionic acid mediated endocytosis and releases melittin intracellularly upon the trigger of elevated redox potential. In vivo study proved that the targeted nanocomplex shows excellent potency in inhibiting tumor growth in a xenograft colon cancer mouse model. Thus, the polymer/melittin nanocomplexes will provide a new approach for melittin based cancer therapy.

Correlation between Single-Molecule Dynamics and Biological Functions of Antimicrobial Peptide Melittin

Many fundamental biological processes occur on cell membranes, and a typical example is the membrane permeabilization by peptides for an antimicrobial purpose. Previous studies of the underlying mechanism mostly focus on structural changes of membranes and peptides during their interactions. Herein, from a new perspective of single-molecule dynamics, the real-time three-dimensional motions of individual phospholipid and peptide molecules were monitored, and specifically, their correlation with the membrane poration function of melittin, a most representative natural antimicrobial peptide, was studied. We found that the adsorption and accumulation of melittin on the membrane surface significantly sped up the lateral diffusion of lipids surrounding the peptides, which in turn facilitated the peptide insertion at such heterogeneous regions. A unique "U"-bending pathway of melittin during membrane insertion and the ultimate formation of toroidal pores with dynamical translocations of peptides and lipids with several metastable states between the two leaflets of bilayer were observed.

Lipid-specific interactions determine the organization and dynamics of membrane-active peptide melittin

The cell membranes of different cells deviate significantly in lipid compositions and thus provide varying biological environments to modulate the diffusion, organization and the resultant function of biomacromolecules. However, the detailed modulation mechanism remains elusive especially in consideration of the current overuse of the simplified membrane models such as the pure phosphatidylcholine (PC) membrane. In this work, with the typical membrane-active peptide melittin, we demonstrated that a more complicated membrane environment, such as the bacterial (IME) or plasma membrane (PM), would significantly change the organization and dynamics of melittin, by using molecular dynamics simulations as a "computational microscope". It was found that in these membrane systems, adding melittin would cause a varying degree of reduction in the lateral diffusion of lipids due to the different assembly states of peptides. Melittin tended to aggregate to oligomers in the pure PC membrane, mostly as a tetramer or trimer, while in IME or PM, its degree of oligomerization was significantly reduced. More surprisingly, melittin displayed a strong affinity with ganglioside GM3 in PM, leading to the formation of melittin-GM3 nanoclusters, which hindered its diffusion and further oligomerization. Additionally, small changes in the residue sequence of melittin could modulate the degree or structure of the peptide oligomer. Our work provides a typical example of a study on the organization and dynamics of pore-forming peptides in specific membrane environments and has great significance on the optimization of peptide sequences and the design of helix bundles in the membrane for target biological function.

Selection of Affinity Reagents to Neutralize the Hemolytic Toxicity of Melittin Based on a Self-Assembled Nanoparticle Library

Antibodies are the most common affinity reagents for specific target recognition. However, their applications are limited by high cost and low stability. Thus, seeking substitutes for antibodies is of great significance. In this work, we designed a library containing 82 self-assembled nanoparticles (SNPs) based on the self-assembly of β-cyclodextrin polymers and adamantane derivatives, and then screened out eight types of SNPs capable of suppressing the toxicity of melittin using a hemolytic activity neutralization assay. The affinities of the SNPs to melittin were demonstrated using surface plasmon resonance (SPR). As evidenced by cytotoxicity experiments, SNPs could also suppress the toxicity of melittin to other cells. In addition, to verify the universality of our method, 11 types of SNPs capable of neutralizing another toxic peptide, phenolic soluble polypeptide (PSMα3) secreted by Staphylococcus aureus, were selected from the same SNP library. Our self-assembly-based method for the library preparation has the advantages of flexible design, mild experimental condition, and simple operation, which is expected to seek artificial affinity reagents for more species.

Graphene oxide-based magnetic nanocomposites for the delivery of melittin to cervical cancer HeLa cells

Melittin (MEL), the primary active component of bee venom, has recently emerged as a promising cancer chemotherapeutic agent. However, the instability and rapid degradation of MEL is a significant challenge in practical therapeutic applications. In the present study, graphene oxide (GO)-based magnetic nanocomposites (PEG-GO-Fe₃O₄) were prepared and adopted as the drug delivery vehicles of MEL, and the anticancer effects of PEG-GO-Fe₃O₄/MEL complexes on human cervical cancer HeLa cells were studied. PEG-GO-Fe₃O₄ exhibited a series of unique physical and chemical properties resulting in multiple interactions with MEL, and ultimately the release of MEL. In vitro experiments showed that PEG-GO-Fe₃O₄/MEL not only distinctly enhanced the inhibition effect on HeLa cells, but also induced pore formation in the cell membrane that ultimately led to cell lysis. In this newly developed drug delivery system, PEGylated GO plays the role of a MEL protector while Fe₃O₄ nanoparticles act as magnetic responders; therefore active MEL can be released over a long period of time (up to 72 h) and maintain its inhibition effect on HeLa cells.

Revisiting the Interaction of Melittin with Phospholipid Bilayers: The Effects of Concentration and Ionic Strength

Melittin is an anti-microbial peptide (AMP) and one of the most studied membrane-disrupting peptides. There is, however, a lack of accurate measurements of the concentration-dependent kinetics and affinity of binding of melittin to phospholipid membranes. In this study, we used surface plasmon resonance spectroscopy to determine the concentration-dependent effect on the binding of melittin to 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) bilayers in vesicles. Three concentration ranges were considered, and when combined, covered two orders of magnitudes (0.04 µM to 8 µM), corresponding to concentrations relevant to the membrane-disrupting and anti-microbial activities of melittin. Binding kinetics data were analysed using a 1:1 Langmuir-binding model and a two-state reaction model. Using in-depth quantitative analysis, we characterised the effect of peptide concentration, the addition of NaCl at physiological ionic strength and the choice of kinetic binding model on the reliability of the calculated kinetics and affinity of binding parameters. The apparent binding affinity of melittin for POPC bilayers was observed to decrease with increasing peptide/lipid (P/L) ratio, primarily due to the marked decrease in the association rate. At all concentration ranges, the two-state reaction model provided a better fit to the data and, thus, a more reliable estimate of binding affinity. Addition of NaCl significantly reduced the signal response during the association phase; however, no substantial effect on the binding affinity of melittin to the POPC bilayers was observed. These findings based on POPC bilayers could have important implications for our understanding of the mechanism of action of melittin on more complex model cell membranes of higher physiological relevance.

Co-delivery of Bee Venom Melittin and a Photosensitizer with an Organic-Inorganic Hybrid Nanocarrier for Photodynamic Therapy and Immunotherapy

Photodynamic therapy (PDT) is a clinical cancer treatment modality based on the induction of therapeutic reactive oxygen species (ROS), which can trigger immunogenic cell death (ICD). With the aim of simultaneously improving both PDT-mediated intracellular ROS production and ICD levels, we designed a serum albumin (SA)-coated boehmite ("B"; aluminum hydroxide oxide) organic-inorganic scaffold that could be loaded with chlorin e6 (Ce6), a photosensitizer, and a honey bee venom melittin (MLT) peptide, denoted Ce6/MLT@SAB. Ce6/MLT@SAB was anchored by a boehmite nanorod structure and exhibited particle size of approximately 180 nm. Ce6/MLT@SAB could significantly reduce hemolysis relative to that of free MLT, while providing MLT-enhanced PDT antitumor effects in vitro. Compared with Ce6@SAB, Ce6/MLT@SAB improved Ce6 penetration of cancer cells both in vitro and in vivo, thereby providing enhanced intracellular ROS generation with 660 nm light treatment. Following phototreatment, Ce6/MLT@SAB-treated cells displayed significantly improved levels of ICD and abilities to activate dendritic cells. In the absence of laser irradiation, multidose injection of Ce6/MLT@SAB could delay the growth of subcutaneous murine tumors by more than 60%, compared to controls. When combined with laser irradiation, a single injection and phototreatment with Ce6/MLT@SAB eradicated one-third of subcutaneous tumors in treated mice. The addition of an immune checkpoint blockade to Ce6/MLT@SAB phototreatment further augmented antitumor effects, generating increased numbers of CD4⁺ and CD8⁺ T cells in tumors with concomitant reduction of myeloid-derived suppressor cells.

Charge-dependent modulation of specific and nonspecific protein-metal ion interactions in nanoelectrospray ionization mass spectrometry

RATIONALE

Previous studies found that charge state could affect both specific and nonspecific binding of protein-metal ion interactions in nanoelectrospray ionization mass spectrometry (nESI-MS). However, the two kinds of interactions have been studied individually in spite of the problem that they often coexist in the same system. Thus, it is necessary to study the effects of charge state on specific and nonspecific protein-metal ion interactions in one system to reveal more accurate binding state.

METHODS

The HIV-1 nucleocapsid protein (NCp7(31-55)) which can bind specifically and nonspecifically to Zn²⁺ served as the model to show the charge-dependent protein-metal ion interactions. Hydrogen/deuterium exchange (HDX) and photodissociation (PD) were used to demonstrate that specific binding state was correlated with protein structure. In addition to NCp7(31-55), three other model proteins were used to investigate the reason for the charge-dependent nonspecific binding.

RESULTS

For specific binding, we proposed that protein ions with different charge states had different conformations. The HDX results showed that labile protons in the NCp7(31-55)-Zn complex were exchanged in a charge-state-dependent way. The PD experiments revealed differential fragment yields for different charge states. For nonspecific binding, higher charge states had more Zn²⁺ additions, but less SO₄ ^2- additions. The effects of charge states on nonspecific binding levels were entirely the opposite for Zn²⁺ and SO₄ ^2- . These results could reveal that the nonspecific binding was caused by electrostatic interaction.

CONCLUSIONS

For specific binding, NCp7(31-55) with lower charge states have folding and undenatured structures. The binding states of lower charge states can better reflect more native binding states. For nonspecific binding, when multiple metal ions adduct to proteins, the proteins have more net positive charges, which tend to generate higher charge ions during electrospray.

Peripheral Antimicrobial Peptide Gomesin Induces Membrane Protrusion, Folding, and Laceration

Optical microscopy shows that the peripheral antimicrobial peptide (AMP) gomesin does not disrupt the bacterial membrane by forming stable transmembrane pores but induces lipid accumulation domains, which is followed by a sudden burst near the domains. The molecular action mechanisms of gomesin on vesicle and planar bilayer membranes are investigated in this work using coarse-grained molecular dynamics simulations. By comparing the membrane morphology and property changes induced by gomesin and the pore-forming AMP melittin, we determined that the amphiphilic shape of the AMPs is a key factor affecting the mechanism of cell death. The binding of wedge-shaped gomesin, with a small hydrophobic surface, onto the membrane induces protrusion and folding of the outer monolayer followed by sudden membrane lacerations at the axillae of the protuberances. Alternatively, cylinder-shaped melittins with comparable hydrophilic and hydrophobic surfaces destroy membranes by forming stable pores coexisting with exocytosis-like buddings and endocytosis-like invaginations. The multiple actions of AMPs on the bacterial membrane suggest diverse paradigms for designing molecular carriers for delivering drugs to the cell.

Self-Assembled Peptide Nanofibers Display Natural Antimicrobial Peptides to Selectively Kill Bacteria without Compromising Cytocompatibility

One of the major hurdles in the development of antimicrobial peptide (AMP)-based materials is their poor capacity in selectively killing bacteria without harming nearby mammalian cells. Namely, they are antimicrobial but cytotoxic. Current methods of nanoparticle-encapsulated AMPs to target bacteria selectively still have not yet overcome this hurdle. Here, we demonstrate a simple yet effective method to address this daunting challenge by associating a natural AMP with a β-sheet-forming synthetic peptide. The integrated peptides self-assembled to form a supramolecular nanofiber, resulting in the presentation of the AMP at the nanofiber-solvent interface in a precisely controlled manner. Using melittin as a model natural AMP, we found that the conformation of melittin changed dramatically when presented on the nanofiber surface, which, in turn, modulated the induced membrane permeability of the bacterial and mammalian cell membranes. Specifically, the presentation of melittin on the nanofiber restricted its hydrophobic residues, leading to a reduction of the hydrophobic interaction with lipids in the cell membranes. Compellingly, the reduced hydrophobic interaction led to a considerable decrease of melittin's induced permeability of the mammalian cell membrane than that of the bacterial cell membrane. As a result, the AMP-displaying nanofiber preferentially permeabilized and disrupted the membrane of the bacteria without compromising the mammalian cells. Such improved membrane selectivity and cytocompatibility were confirmed in a cell-based membrane localization and live-dead assay. Our new strategy holds great promise for fabricating cytocompatible antimicrobial assemblies that offer safer and more effective administration of therapeutic AMPs. These assemblies, with intrinsic antimicrobial activity and cytocompatibility, can also serve as building blocks for the construction of higher-ordered scaffolds for other biomedical applications such as tissue engineering and regenerative medicine.

Co-delivery of paclitaxel and melittin by glycopeptide-modified lipodisks for synergistic anti-glioma therapy

Nanosized lipodisks with flat circular phospholipid bilayers surrounded by PEGylated edges have demonstrated promise in drug delivery. In the present work, a lipodisk-based paclitaxel and melittin co-delivery system functionalized with glycopeptide 9G-A7R (9G-A7R-Disk/PTX/melittin) was successfully constructed, in which melittin which was fully protected from proteolysis and hemolysis was effectively reduced. The ratio of paclitaxel to melittin in lipodisks could be accurately controlled through a film hydration-adsorption method. Paclitaxel combined with melittin showed synergism against U87 cells in vitro, and 9G-A7R-Disk/PTX/melittin demonstrated an enhanced anti-glioma effect in vivo, significantly prolonging the survival time of glioma-bearing mice. The results suggested a promising formulation for the co-delivery of paclitaxel/melittin and glioma-targeted therapy.

How Melittin Inserts into Cell Membrane: Conformational Changes, Inter-Peptide Cooperation, and Disturbance on the Membrane

Antimicrobial peptides (AMPs), as a key component of the immune defense systems of organisms, are a promising solution to the serious threat of drug-resistant bacteria to public health. As one of the most representative and extensively studied AMPs, melittin has exceptional broad-spectrum activities against microorganisms, including both Gram-positive and Gram-negative bacteria. Unfortunately, the action mechanism of melittin with bacterial membranes, especially the underlying physics of peptide-induced membrane poration behaviors, is still poorly understood, which hampers efforts to develop melittin-based drugs or agents for clinical applications. In this mini-review, we focus on recent advances with respect to the membrane insertion behavior of melittin mostly from a computational aspect. Membrane insertion is a prerequisite and key step for forming transmembrane pores and bacterial killing by melittin, whose occurrence is based on overcoming a high free-energy barrier during the transition of melittin molecules from a membrane surface-binding state to a transmembrane-inserting state. Here, intriguing simulation results on such transition are highlighted from both kinetic and thermodynamic aspects. The conformational changes and inter-peptide cooperation of melittin molecules, as well as melittin-induced disturbances to membrane structure, such as deformation and lipid extraction, are regarded as key factors influencing the insertion of peptides into membranes. The associated intermediate states in peptide conformations, lipid arrangements, membrane structure, and mechanical properties during this process are specifically discussed. Finally, potential strategies for enhancing the poration ability and improving the antimicrobial performance of AMPs are included as well.

Free energy analysis of membrane pore formation process in the presence of multiple melittin peptides

Understanding the molecular mechanism underlying pore formation in lipid membranes by antimicrobial peptides is of great importance in biological sciences as well as in drug design applications. Melittin has been widely studied as a pore forming peptide, though the molecular mechanism for pore formation is still illusive. We examined the free energy barrier for the creation of a pore in lipid membranes with and without multiple melittin peptides. It was found that six melittin peptides significantly stabilized a pore, though a small barrier (a few k_BT) for the formation still existed. With five melittin peptides or fewer, the pore formation barrier was much higher, though the established pore was in a local energy minimum. Although seven melittins effectively reduced the free energy barrier, a single melittin peptide left the pore after a long time MD simulation probably because of the overcrowded environment around the bilayer pore. Thus, it is highly selective for the number of melittin peptides to stabilize the membrane pore, as was also suggested by the line tension evaluations. The free energy cost required to insert a single melittin into the membrane is too high to explain the one-by-one insertion mechanism for pore formation, which also supports the collective melittin mechanism for pore formation.

Bioengineered Macrophages Can Responsively Transform into Nanovesicles To Target Lung Metastasis

Specific drug delivery to metastatic tumors remains a great challenge for antimetastasis therapy. We herein report a bioengineered macrophage-based delivery system (LD-MDS) that can be preferentially delivered to lung metastases and intelligently transformed into nanovesicles and secondary nanovesicles for antimetastasis therapy. LD-MDS was prepared by anchoring a legumain-specific propeptide of melittin (legM) and cytotoxic soravtansine (DM4) prodrug onto the membrane of living macrophages. LD-MDS is responsively activated by legumain protease and converted into DM4-loaded exosome-like nanovesicles (DENs), facilitating efficient internalization by metastatic 4T1 cancer cells and considerable cell death. Afterward, the damaged 4T1 cells can release secondary nanovesicles and free drug molecules to destroy neighboring cancer cells. In vivo, LD-MDS displays superior targeting efficiency for lung metastatic lesions with diameters less than 100 μm and remarkably inhibits lung metastasis. This study provides a new opportunity to explore endogenous macrophages as living drug delivery vehicles with controlled drug release to target metastatic lung tumors.

Towards mapping electrostatic interactions between Kdo2-lipid A and cationic antimicrobial peptides via ultraviolet photodissociation mass spectrometry

Cationic antimicrobial peptides (CAMPs) have been known to act as multi-modal weapons against Gram-negative bacteria. As a new approach to investigate the nature of the interactions between CAMPs and the surfaces of bacteria, native mass spectrometry and two MS/MS strategies (ultraviolet photodissociation (UVPD) and higher energy collisional activation (HCD)) are used to examine formation and disassembly of saccharolipid·peptide complexes. Kdo2-lipid A (KLA) is used as a model saccharolipid to evaluate complexation with a series of cationic peptides (melittin and three analogs). Collisional activation of the KLA·peptide complexes results in the disruption of electrostatic interactions, resulting in apo-sequence ions with shifts in the distribution of ions compared to the fragmentation patterns of the apo-peptides. UVPD of the KLA·peptide complexes results in both apo- and holo-sequence ions of the peptides, the latter in which the KLA remains bound to the truncated peptide fragment despite cleavage of a covalent bond of the peptide backbone. Mapping both the N- and C-terminal holo-product ions gives insight into the peptide motifs (specifically an electropositive KRKR segment and a proline residue) that are responsible for mediating the electrostatic interactions between the cationic peptides and saccharolipid.

Conformations and Dynamic Transitions of a Melittin Derivative That Forms Macromolecule-Sized Pores in Lipid Bilayers

Systematically evolved from the primary active component of bee venom, MelP5 is a lipophilic peptide with important physical properties that differ from wild-type melittin, including the ability to create large equilibrium pores in lipid bilayers at low peptide to lipid ratios. Self-assembly into stable membrane spanning pores makes MelP5 a promising candidate for future applications in the pharmaceutical arena. Despite significant interest, little is known about the mechanism by which MelP5 remodels the lipid bilayer upon binding. We demonstrate by direct atomic force microscope imaging of supported lipid bilayers in solution that MelP5 remodels 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC) in one of two ways. It creates either highly localized voids in the bilayer or diffuse nonlocalized thinning. Thinning of the bilayer was measured to be 3.0 ± 1.4 Å (mean ± standard deviation) below the surface of the upper leaflet of the bilayer. Pores, defined as highly localized voids in the bilayer, exhibited several sizes. Approximately 20% of pores exhibited large footprint areas (47 ± 20 nm²) which appear capable of passing bulky macromolecules. The peptide-effected bilayer was observed to reversibly exchange between membrane-thinned and pore states in an apparent dynamic equilibrium. Analysis of time-lapsed images suggested upper and lower bounds (0.2 < τ < 180 s) on the characteristic time scale of transitions between the membrane-thinned and pore states. Moreover, pores were found to colocalize with membrane-thinned regions, a novel observation that is consistent with the notion of cooperativity among membrane-bound peptides when forming pores.

Nanoscale Melittin@Zeolitic Imidazolate Frameworks for Enhanced Anticancer Activity and Mechanism Analysis

The cytolytic peptide melittin (MLT) is an important candidate of anticancer drug owing to its hemolytic properties. Nevertheless, its clinical applications are severely restricted as a result of its nonspecific toxicities like hemolysis. In this work, we reported MLT-loaded zeolitic imidazolate framework-8 (MLT@ZIF-8) nanoparticles (NPs). The formed MLT@ZIF-8 NPs not only possess excellent stability but also efficiently inhibit the hemolysis bioactivity of MLT. Confocal scanning imaging and cytotoxicity experiments revealed that as-synthesized MLT@ZIF-8 NPs exhibit enhanced cellular uptake and cytotoxicity toward cancer cells compared to MLT. The mechanism is well investigated by a series of transcriptome analysis, which indicates that MLT@ZIF-8 NPs can regulate the expression of 3383 genes, and the PI3K/Akt-regulated p53 pathway is involved in MLT@ZIF-8 NPs induced A549 cells apoptosis. Finally, MLT@ZIF-8 NPs exhibit enhanced antitumor activity than free MLT in vivo, while no obvious systemic toxicity has been found. This work emphasizes the great potential of utilizing MOF as a simple and efficient nanoplatform for deliverying cytolytic peptides in cancer treatment, and also the investigation on the antitumor mechanism could provide theoretical support for clinical usage of MLT.

Broad-Spectrum Neutralization of Pore-Forming Toxins with Human Erythrocyte Membrane-Coated Nanosponges

Neutralization of bacterial toxins has become a compelling approach to treating bacterial infections as it may pose less selective pressure for the development of bacterial resistance. Currently, the majority of toxin neutralization platforms act by targeting the molecular structure of the toxin, which requires toxin identification and customized design for different diseases. Therefore, their development has been challenged by the enormous number and complexity of bacterial toxins. Herein, biomimetic toxin nanosponges are formulated by coating membranes of human red blood cells (hRBCs) onto polymeric nanoparticles, which act as a toxin decoy to absorb and neutralize a broad-spectrum of hemolytic toxins regardless of their molecular structure. When tested with model pore-forming toxins, including melittin, α-hemolysin of methicillin-resistant Staphylococcus aureus, listeriolysin O of Listeria monocytogenes, and streptolysin O of Group A Streptococcus, the hRBC nanosponges are able to completely inhibit toxin-induced hemolysis in a concentration-dependent manner. In addition, the nanosponge-detained toxins show no cytotoxicity when tested on human umbilical vein endothelial cells and no lethality when injected into mice, which together indicate effective toxin neutralization. Overall, these results demonstrate the broad applicability and high effectiveness of the hRBC nanosponges as a novel antivirulence platform against hemolytic toxins from various strains of bacteria.

Unravelling a Mechanism of Action for a Cecropin A-Melittin Hybrid Antimicrobial Peptide: The Induced Formation of Multilamellar Lipid Stacks

An understanding of the mechanism of action of antimicrobial peptides is fundamental to the development of new and more active antibiotics. In the present work, we use a wide range of techniques (SANS, SAXD, DSC, ITC, CD, and confocal and electron microscopy) in order to fully characterize the interaction of a cecropin A-melittin hybrid antimicrobial peptide, CA(1-7)M(2-9), of known antimicrobial activity, with a bacterial model membrane of POPE/POPG in an effort to unravel its mechanism of action. We found that CA(1-7)M(2-9) disrupts the vesicles, inducing membrane condensation and forming an onionlike structure of multilamellar stacks, held together by the intercalated peptides. SANS and SAXD revealed changes induced by the peptide in the lipid bilayer thickness and the bilayer stiffening in a tightly packed liquid-crystalline lamellar phase. The analysis of the observed abrupt changes in the repeat distance upon the phase transition to the gel state suggests the formation of an L_γ phase. To the extent of our knowledge, this is the first time that the L_γ phase is identified as part of the mechanism of action of antimicrobial peptides. The energetics of interaction depends on temperature, and ITC results indicate that CA(1-7)M(2-9) interacts with the outer leaflet. This further supports the idea of a surface interaction that leads to membrane condensation and not to pore formation. As a result, we propose that this peptide exerts its antimicrobial action against bacteria through extensive membrane disruption that leads to cell death.

Bicontinuous microemulsions as a biomembrane mimetic system for melittin

Antimicrobial peptides effectively kill antibiotic-resistant bacteria by forming pores in prokaryotes' biomembranes via penetration into the biomembranes' interior. Bicontinuous microemulsions, consisting of interdispersed oil and water nanodomains separated by flexible surfactant monolayers, are potentially valuable for hosting membrane-associated peptides and proteins due to their thermodynamic stability, optical transparency, low viscosity, and high interfacial area. Here, we show that bicontinuous microemulsions formed by negatively-charged surfactants are a robust biomembrane mimetic system for the antimicrobial peptide melittin. When encapsulated in bicontinuous microemulsions formed using three-phase (Winsor-III) systems, melittin's helicity increases greatly due to penetration into the surfactant monolayers, mimicking its behavior in biomembranes. But, the threshold melittin concentration required to achieve these trends is lower for the microemulsions. The extent of penetration was decreased when the interfacial fluidity of the microemulsions was increased. These results suggest the utility of bicontinuous microemulsions for isolation, purification, delivery, and host systems for antimicrobial peptides.

Melittin-Containing Hybrid Peptide Hydrogels for Enhanced Photothermal Therapy of Glioblastoma

The design of biocompatible and efficacious anticancer biomaterials to achieve relatively low tumor recurrence rates is the main pursuit of cancer photothermal therapy (PTT). RADA16-I is a synthetic amphiphilic peptide with the sequence RADARADARADARADA that can self-assemble into a peptide nanofiber hydrogel. In this study, we synthesized a novel melittin-RADA₃₂-indocyanine green (ICG) hydrogel ("MRI hydrogel"), which contains melittin in the peptide hydrogel backbone and ICG in the hydrogel matrix, for enhanced PTT of glioblastomas. The MRI hydrogel exhibited physiologic characteristics similar to those of the RADA₁₆ hydrogel, while displaying concentration-dependent cytotoxicity to C6 glioma cells and photothermal effects. The in vivo biodistribution of the MRI hydrogel was visualized by near-infrared fluorescence and photoacoustic imaging. More importantly, in vivo PTT provided by the MRI hydrogel significantly reduced the tumor size and the tumor recurrence rate compared with the RADR₁₆-ICG hydrogel and other controls, suggesting a synergistic effect of MRI hydrogel-carried melittin and ICG-based PTT treatment. Thus, MRI provides an alternative tool for the safe and efficient PTT treatment of tumors.

A New Mixed All-Atom/Coarse-Grained Model: Application to Melittin Aggregation in Aqueous Solution

We introduce a new mixed resolution, all-atom/coarse-grained approach (AACG), for modeling peptides in aqueous solution and apply it to characterizing the aggregation of melittin. All of the atoms in peptidic components are represented, while a single site is used for each water molecule. With the full flexibility of the peptide retained, our AACG method achieves speedups by a factor of 3-4 for CPU time reduction and another factor of roughly 7 for diffusion. An Ewald treatment permits the inclusion of long-range electrostatic interactions. These characteristics fit well with the requirements for studying peptide association and aggregation, where the system sizes and time scales require considerable computational resources with all-atom models. In particular, AACG is well suited for biologics since changes in peptide shape and long-range electrostatics may play an important role. The application of AACG to melittin, a 26-residue peptide with a well-known propensity to aggregate in solution, serves as an initial demonstration of this technology for studying peptide aggregation. We observed the formation of melittin aggregates during our simulations and characterized the time-evolution of aggregate size distribution, buried surface areas, and residue contacts. Key interactions including π-cation and π-stacking involving TRP19 were also examined. Our AACG simulations demonstrated a clear salt effect and a moderate temperature effect on aggregation and support the molten globule model of melittin aggregates. As a showcase, this work illustrates the useful role for AACG in investigations of peptide aggregation and its potential to guide formulation and design of biologics.

Interactions of the antimicrobial peptide nisin Z with conventional antibiotics and the use of nanostructured lipid carriers to enhance antimicrobial activity

Antimicrobial resistance is an imminent threat to the effective prevention and treatment of bacterial infections and alternative antimicrobial strategies are desperately needed. Antimicrobial peptides (AMPs) may be promising alternatives to current antibiotics or act as adjuvants to enhance antibiotic potency. Additionally, the use of biodegradable lipid nanoparticles can enhance the antibacterial activity of antibiotics and antimicrobial peptides. In this study, the interaction of the AMPs, nisin Z and melittin, with conventional antibiotics was investigated on Staphylococcus aureus, Staphylococcus epidermidis and Escherichia coli. The effectiveness of nanostructured lipid carriers (NLCs) for the entrapment of nisin Z was also evaluated. Findings revealed that nisin Z exhibited additive interactions with numerous conventional antibiotics. Notable synergism was observed for novobiocin-nisin Z combinations. The addition of the non-antibiotic adjuvant EDTA significantly improved the antimicrobial activity of free nisin Z towards E.coli. NLCs containing nisin Z were effective against Gram-positive species at physiological pH, with an increase in effectiveness in the presence of EDTA. Results indicate that nisin Z may be advantageous as an adjuvant in antimicrobial chemotherapy, while contributing in the battle against antibiotic resistance. NLCs have the potential to enhance the antibacterial activity of nisin Z towards Gram-positive bacterial species associated with skin infections.

Characterizing and Controlling the Loading and Release of Cationic Amphiphilic Peptides onto and from PEG-Stabilized Lipodisks

Recent studies have identified PEG-stabilized lipid nanodisks (lipodisks) as promising carriers for cationic amphiphilic peptides with antimicrobial and anticancer activity. Using fluorimetric and nanogravimetric methods, we have in this work characterized the parameters describing and controlling the binding of three selected peptides (melittin, LL37, and magainin 2) onto lipodisks. It was found that the affinity of melittin for lipodisks is independent of the disk size and rim charge. On the other hand, the number of binding sites is strongly dependent on both parameters, with the highest loading being obtained for small disks with a negatively charged rim. An optimized composition of the lipodisks was utilized to study the loading of antimicrobial peptides magainin 2 and human LL37. It was observed that although magainin 2 can be loaded in large amounts, it is released very fast upon dilution, which limits future therapeutic applications. In contrast, LL37 can be loaded at relevant concentrations and the formulation is stable. This opens up for applications of LL37-loaded lipodisks as antibiotics and in anticancer treatments.

Direct Measurement of Pore Dynamics and Leakage Induced by a Model Antimicrobial Peptide in Single Vesicles and Cells

Antimicrobial peptides are promising therapeutic alternatives to counter growing antimicrobial resistance. Their precise mechanism of action remains elusive, however, particularly with respect to live bacterial cells. We investigated the interaction of a fluorescent melittin analogue with single giant unilamellar vesicles, giant multilamellar vesicles, and bilamellar Gram-negative Escherichia coli (E. coli) bacteria. Time-lapse fluorescence lifetime imaging microscopy was employed to determine the population distribution of the fluorescent melittin analogue between pore state and membrane surface state, and simultaneously measure the leakage of entrapped fluorescent species from the vesicle (or bacterium) interior. In giant unilamellar vesicles, leakage from vesicle interior was correlated with an increase in level of pore states, consistent with a stable pore formation mechanism. In giant multilamellar vesicles, vesicle leakage occurred more gradually and did not appear to correlate with increased pore states. Instead pore levels remained at a low steady-state level, which is more in line with coupled equilibria. Finally, in single bacterial cells, significant increases in pore levels were observed over time, which were correlated with only partial loss of cytosolic contents. These observations suggested that pore formation, as opposed to complete dissolution of membrane, was responsible for the leakage of contents in these systems, and that the bacterial membrane has an adaptive capacity that resists peptide attack. We interpret the three distinct pore dynamics regimes in the context of the increasing physical and biological complexity of the membranes.

Identification of a Region of the Polypeptide Chain of Na,K-ATPase α-Subunit Interacting with 67-kDa Melittin-Like Protein

It was shown earlier that a 67-kDa protein purified from mouse kidney using polyclonal antibodies against melittin (a peptide from bee venom) interacted with Na,K-ATPase from rabbit kidney. In this study, a 43-kDa proteolytic fragment of Na,K-ATPase α-subunit interacting with the 67-kDa melittin-like protein was found. The α-subunit was hydrolyzed by trypsin in the presence of 0.5 mM ouabain (E2-conformation of Na,K-ATPase). A proteolytic fragment interacting with the 67-kDa melittin-like protein that was identified by mass-spectrometry is a region of the cytoplasmic domain of Na,K-ATPase α-subunit located between amino acid residues 591 and 775. The fragment includes a conservative DPPRA motif that occurs in many P-type ATPases. It was shown earlier that this motif of H,K-ATPase from gastric mucosa binds to melittin. We suggest that namely this motif of P-type ATPases is able to interact with proteins containing melittin-like modules.

Multistep Molecular Dynamics Simulations Identify the Highly Cooperative Activity of Melittin in Recognizing and Stabilizing Membrane Pores

The prototypical antimicrobial peptide, melittin, is well-known for its ability to induce pores in zwitterionic model lipid membranes. However, the mechanism by which melittin accomplishes this is not fully understood. We have conducted all-atom and coarse-grained molecular dynamics simulations which suggest that melittin employs a highly cooperative mechanism for the induction of both small and large membrane pores. The process by which this peptide induces membrane pores appears to be driven by its affinity to membrane defects via its N-terminus region. In our simulations, a membrane defect was deliberately created through either lipid flip-flop or the reorientation of one adsorbed melittin peptide. In a cooperative response, other melittin molecules also inserted their N-termini into the created defect, thus lowering the overall free energy. The insertion of these peptide molecules ultimately allowed the defect to develop into a small transmembrane pore, with an estimated diameter of ∼1.5 nm and a lifetime of the order of tens of milliseconds. In the presence of a finite membrane tension, we show that this small pore can act as a nucleation site for the stochastic rupture of the lipid bilayer, so as to create a much larger pore. We found that a threshold membrane tension of 25 mN/m was needed to create a ruptured pore. Furthermore, by actively accumulating at its edge, adsorbed peptides are able to cooperatively stabilize this larger pore. The defect-mediated pore formation mechanism revealed in this work may also apply to other amphipathic membrane-active peptides.

Site of fluorescent label modifies interaction of melittin with live cells and model membranes

The mechanism of membrane disruption by melittin (MLT) of giant unilamellar vesicles (GUVs) and live cells was studied using fluorescence microscopy and two fluorescent synthetic analogues of MLT. The N-terminus of one of these was acylated with thiopropionic acid to enable labeling with maleimido-AlexaFluor 430 to study the interaction of MLT with live cells. It was compared with a second analogue labeled at P14C. The results indicated that the fluorescent peptides adhered to the membrane bilayer of phosphatidylcholine GUVs and inserted into the plasma membrane of HeLa cells. Fluorescence and light microscopy revealed changes in cell morphology after exposure to MLT peptides and showed bleb formation in the plasma membrane of HeLa cells. However, the membrane disruptive effect was dependent upon the location of the fluorescent label on the peptide and was greater when MLT was labeled at the N-terminus. Proline at position 14 appeared to be important for antimicrobial activity, hemolysis and cytotoxicity, but not essential for cell membrane disruption.

Defined nanoscale chemistry influences delivery of peptido-toxins for cancer therapy

We present an in-silico-to-in-vitro approach to develop well-defined, self-assembled, rigid-cored polymeric (Polybee) nano-architecture for controlled delivery of a key component of bee venom, melittin. A competitive formulation with lipid-encapsulated (Lipobee) rigid cored micelle is also synthesized. In a series of sequential experiments, we show how nanoscale chemistry influences the delivery of venom toxins for cancer regression and help evade systemic disintegrity and cellular noxiousness. A relatively weaker association of melittin in the case of lipid-based nanoparticles is compared to the polymeric particles revealed by energy minimization and docking studies, which are supported by biophysical studies. For the first time, the authors' experiment results indicate that melittin can play a significant role in DNA association-dissociation processes, which may be a plausible route for their anticancer activity.

Cytotoxic helix-rich oligomer formation by melittin and pancreatic polypeptide

Conversion of amyloid fibrils by many peptides/proteins involves cytotoxic helix-rich oligomers. However, their toxicity and biophysical studies remain largely unknown due to their highly dynamic nature. To address this, we chose two helical peptides (melittin, Mel and pancreatic polypeptide, PP) and studied their aggregation and toxicity. Mel converted its random coil structure to oligomeric helical structure upon binding to heparin; however, PP remained as helix after oligomerization. Interestingly, similar to Parkinson’s associated α-synuclein (AS) oligomers, Mel and PP also showed tinctorial properties, higher hydrophobic surface exposure, cellular toxicity and membrane pore formation after oligomerization in the presence of heparin. We suggest that helix-rich oligomers with exposed hydrophobic surface are highly cytotoxic to cells irrespective of their disease association. Moreover as Mel and PP (in the presence of heparin) instantly self-assemble into stable helix-rich amyloidogenic oligomers; they could be represented as models for understanding the biophysical and cytotoxic properties of helix-rich intermediates in detail.