Cyclic tachyplesin I kills proliferative, non-proliferative and drug-resistant melanoma cells without inducing resistance

Acquired drug resistance is the major cause for disease recurrence in cancer patients, and this is particularly true for patients with metastatic melanoma that carry a BRAF V600E mutation. To address this problem, we investigated cyclic membrane-active peptides as an alternative therapeutic modality to kill drug-tolerant and resistant melanoma cells to avoid acquired drug resistance. We selected two stable cyclic peptides (cTI and cGm), previously shown to have anti-melanoma properties, and compared them with dabrafenib, a drug used to treat cancer patients with the BRAF V600E mutation. The peptides act via a fast membrane-permeabilizing mechanism and kill metastatic melanoma cells that are sensitive, tolerant, or resistant to dabrafenib. Melanoma cells do not become resistant to long-term treatment with cTI, nor do they evolve their lipid membrane composition, as measured by lipidomic and proteomic studies. In vivo studies in mice demonstrated that the combination treatment of cTI and dabrafenib resulted in fewer metastases and improved overall survival. Such cyclic membrane-active peptides are thus well suited as templates to design new anticancer therapeutic strategies.

Peptide-Mediated Nanocarriers for Targeted Drug Delivery: Developments and Strategies

Effective drug delivery is essential for cancer treatment. Drug delivery systems, which can be tailored to targeted transport and integrated tumor therapy, are vital in improving the efficiency of cancer treatment. Peptides play a significant role in various biological and physiological functions and offer high design flexibility, excellent biocompatibility, adjustable morphology, and biodegradability, making them promising candidates for drug delivery. This paper reviews peptide-mediated drug delivery systems, focusing on self-assembled peptides and peptide-drug conjugates. It discusses the mechanisms and structural control of self-assembled peptides, the varieties and roles of peptide-drug conjugates, and strategies to augment peptide stability. The review concludes by addressing challenges and future directions.

Resistance is futile: targeting multidrug-resistant bacteria with de novo Cys-rich cyclic polypeptides

The search for novel antimicrobial agents to combat microbial pathogens is intensifying in response to rapid drug resistance development to current antibiotic therapeutics. The use of disulfide-rich head-to-tail cyclized polypeptides as molecular frameworks for designing a new type of peptide antibiotics is gaining increasing attention among the scientific community and the pharmaceutical industry. The use of macrocyclic peptides, further constrained by the presence of several disulfide bonds, makes these peptide frameworks remarkably more stable to thermal, biological, and chemical degradation showing better activities when compared to their linear analogs. Many of these novel peptide scaffolds have been shown to have a high tolerance to sequence variability in those residues not involved in disulfide bonds, able to cross biological membranes, and efficiently target complex biomolecular interactions. Hence, these unique properties make the use of these scaffolds ideal for many biotechnological applications, including the design of novel peptide antibiotics. This article provides an overview of the new developments in the use of several disulfide-rich cyclic polypeptides, including cyclotides, θ-defensins, and sunflower trypsin inhibitor peptides, among others, in the development of novel antimicrobial peptides against multidrug-resistant bacteria.

Native and Engineered Cyclic Disulfide-Rich Peptides as Drug Leads

Bioactive peptides are a highly abundant and diverse group of molecules that exhibit a wide range of structural and functional variation. Despite their immense therapeutic potential, bioactive peptides have been traditionally perceived as poor drug candidates, largely due to intrinsic shortcomings that reflect their endogenous heritage, i.e., short biological half-lives and poor cell permeability. In this review, we examine the utility of molecular engineering to insert bioactive sequences into constrained scaffolds with desired pharmaceutical properties. Applying lessons learnt from nature, we focus on molecular grafting of cyclic disulfide-rich scaffolds (naturally derived or engineered), shown to be intrinsically stable and amenable to sequence modifications, and their utility as privileged frameworks in drug design.

Unlocking the Potential of the Antimicrobial Peptide Gomesin: From Discovery and Structure-Activity Relationships to Therapeutic Applications

Gomesin is a cationic antimicrobial peptide which is isolated from the haemocytes of the Brazilian tarantula Acanthoscurria gomesiana and can be produced chemically by Fmoc solid-phase peptide synthesis. Gomesin exhibits a range of biological activities, as demonstrated by its toxicity against therapeutically relevant pathogens such as Gram-positive or Gram-negative bacteria, fungi, cancer cells, and parasites. In recent years, a cyclic version of gomesin has been used for drug design and development as it is more stable than native gomesin in human serum and can penetrate and enter cancer cells. It can therefore interact with intracellular targets and has the potential to be developed as a drug lead for to treat cancer, infectious diseases, and other human diseases. This review provides a perspective on the discovery, structure-activity relationships, mechanism of action, biological activity, and potential clinical applications of gomesin.

Butelase 1-Mediated Enzymatic Cyclization of Antimicrobial Peptides: Improvements on Stability and Bioactivity

Antimicrobial peptides (AMPs) have broad-spectrum antibacterial properties and safety as food preservatives, whereas the stability and antibacterial activity require improvement. Here, the "head-to-tail" cyclization of linear AMP GKE was catalyzed by butelase 1, which resulted in an improved pronouncedly antibacterial effect. Cell morphology and propidium iodide uptake revealed that the increased membrane permeability was one of the bacteriostatic mechanisms of GKE and could be enhanced after cyclization. As cyclic GKE (cGKE) exhibited more stability than the linear counterpart under the microorganism culture environment, the increase in effective bacteriostatic concentration should be a reason for the superior antibacterial effect. Moreover, cGKE exhibited the ordered secondary structure, while GKE possessed a similar structure only in sodium dodecyl sulfate micelles. The structure was also beneficial to improve the antibacterial activity caused by the increased affinity of cGKE to the membranes. Overall, butelase 1-mediated cyclization is a promising strategy for enhancing the antibacterial activity of linear AMPs.

Peptide-based LDH5 inhibitors enter cancer cells and impair proliferation

Lactate dehydrogenase 5 (LDH5) is overexpressed in many cancers and is a potential target for anticancer therapy due to its role in aerobic glycolysis. Small-molecule drugs have been developed as competitive inhibitors to bind substrate/cofactor sites of LDH5, but none reached the clinic to date. Recently, we designed the first LDH5 non-competitive inhibitor, cGmC9, a peptide that inhibits protein-protein interactions required for LDH5 enzymatic activity. Peptides are gaining a large interest as anticancer agents to modulate intracellular protein-protein interactions not targetable by small molecules; however, delivery of these peptides to the cytosol, where LDH5 and other anticancer targets are located, remains a challenge for this class of therapeutics. In this study, we focused on the cellular internalisation of cGmC9 to achieve LDH5 inhibition in the cytosol. We designed cGmC9 analogues and compared them for LDH5 inhibition, cellular uptake, toxicity, and antiproliferation against a panel of cancer cell lines. The lead analogue, [R/r]cGmC9, specifically impairs proliferation of cancer cell lines with high glycolytic profiles. Proteomics analysis showed expected metabolic changes in response to decreased glycolysis. This is the first report of a peptide-based LDH5 inhibitor able to modulate cancer metabolism and kill cancer cells that are glycolytic. The current study demonstrates the potential of using peptides as inhibitors of intracellular protein-protein interactions relevant for cancer pathways and shows that active peptides can be rationally designed to improve their cell permeation.

Peptide Carbocycles: From -SS- to -CC- via a Late-Stage "Snip-and-Stitch"

One way to improve the therapeutic potential of peptides is through cyclization. This is commonly done using a disulfide bond between two cysteine residues in the peptide. However, disulfide bonds are susceptible to reductive cleavage, and this can deactivate the peptide and endanger endogenous proteins through covalent modification. Substituting disulfide bonds with more chemically robust carbon-based linkers has proven to be an effective strategy to better develop cyclic peptides as drugs, but finding the optimal carbon replacement is synthetically laborious. We report a new late-stage platform wherein a single disulfide bond in a cyclic peptide can serve as the progenitor for any number of new carbon-rich groups, derived from organodiiodides, using a Zn:Cu couple and a hydrosilane. We show that this platform can furnish entirely new carbocyclic scaffolds with enhanced permeability and structural integrity and that the stereochemistry of the new cycles can be biased by a judicious choice in silane.

A designed cyclic analogue of gomesin has potent activity against Staphylococcus aureus biofilms

BACKGROUND

Infections caused by bacterial biofilms are very difficult to treat. The use of currently approved antibiotics even at high dosages often fails, making the treatment of these infections very challenging. Novel antimicrobial agents that use distinct mechanisms of action are urgently needed.

OBJECTIVES

To explore the use of [G1K,K8R]cGm, a designed cyclic analogue of the antimicrobial peptide gomesin, as an alternative approach to treat biofilm infections.

METHODS

We studied the activity of [G1K,K8R]cGm against biofilms of Staphylococcus aureus, a pathogen associated with several biofilm-related infections. A combination of atomic force and real-time confocal laser scanning microscopies was used to study the mechanism of action of the peptide.

RESULTS

The peptide demonstrated potent activity against 24 h-preformed biofilms through a concentration-dependent ability to kill biofilm-embedded cells. Mechanistic studies showed that [G1K,K8R]cGm causes morphological changes on bacterial cells and permeabilizes their membranes across the biofilm with a half-time of 65 min. We also tested an analogue of [G1K,K8R]cGm without disulphide bonds, and a linear unfolded analogue, and found both to be inactive.

CONCLUSIONS

The results suggest that the 3D structure of [G1K,K8R]cGm and its stabilization by disulphide bonds are essential for its antibacterial and antibiofilm activities. Moreover, our findings support the potential application of this stable cyclic antimicrobial peptide to fight bacterial biofilms.

Discovery of Cyclic Peptide Inhibitors Targeting PD-L1 for Cancer Immunotherapy

Blockade of the interaction between programmed cell death ligand-1 (PD-L1) and its receptor PD-1 has shown great success in cancer immunotherapy. Peptides possess unique characteristics that give them significant advantages as immune checkpoint inhibitors. However, unfavorable physicochemical properties and proteolytic stability profiles limit the translation of bioactive peptides as therapeutic agents. Studies have revealed that cyclization improves the biological activity and stability of linear peptides. In this study, we report the use of macrocyclization scanning for the discovery of cyclic anti-PD-L1 peptides with improved bioactivity. The cyclic peptides demonstrated up to a 34-fold improvement in the PD-1/PD-L1 blocking activity and significant in vivo anti-tumor activity. Our results demonstrate that macrocyclization scanning is an effective way to improve the serum stability and bioactivity of the anti-PD-L1 linear peptide. This strategy can be employed in the optimization of other bioactive peptides, particularly those for protein-protein interaction modulation.

Antimicrobial Peptides with Anti-Candida Activity

Mycoses are accountable for millions of infections yearly worldwide. Invasive candidiasis is the most usual, presenting a high morbidity and mortality. Candida albicans remains the prevalent etiologic agent, but the incidence of other species such as Candida parapsilosis, Candida glabrata and Candida auris keeps increasing. These pathogens frequently show a reduced susceptibility to commonly used antifungal drugs, including polyenes, triazoles and echinocandins, and the incidence of emerging multi-drug-resistant strains of these species continues to increase. Therefore, the need to search for new molecules that target these pathogenic species in a different manner is now more urgent than ever. Nature is an almost endless source of interesting new molecules that could meet this need. Among these molecules, antimicrobial peptides, present in different sources in nature, possess some advantages over conventional antifungal agents, even with their own drawbacks, and are considered as a promising pharmacological option against a wide range of microbial infections. In this review, we describe 20 antimicrobial peptides from different origins that possess an activity against Candida.

The dual interaction of antimicrobial peptides on bacteria and cancer cells; mechanism of action and therapeutic strategies of nanostructures

Microbial infection and cancer are two leading causes of global mortality. Discovering and developing new therapeutics with better specificity having minimal side-effects and no drug resistance are of an immense need. In this regard, cationic antimicrobial peptides (AMP) with dual antimicrobial and anticancer activities are the ultimate choice. For better efficacy and improved stability, the AMPs available for treatment still required to be modified. There are several strategies in which AMPs can be enhanced through, for instance, nano-carrier application with high selectivity and specificity enables researchers to estimate the rate of drug delivery to a particular tissue. In this review we present the biology and modes of action of AMPs for both anticancer and antimicrobial activities as well as some modification strategies to improve the efficacy and selectivity of these AMPs.

Strategies to Improve the Activity and Biocompatibility: Modification of Peptide Antibiotics

As host defense peptides, peptide antibiotics exist in almost all organisms. Many of their activities come from their inactivation of bacteria, yeast, fungi, and even cancer cells. However, natural peptide antibiotics are relatively poor in stability and penetration, and have high hemolytic properties, which makes them difficult to directly apply. Therefore, natural peptide antibiotics can be modified to enhance their activity and biocompatibility. Based on the characteristics of amino acids, amino acid substitutions can be performed to study the effect of amino acid types on the activity of peptide antibiotics. The design of ultrashort peptides, cyclic peptides, and self-assembling peptides is also a way to improve the activity of peptide antibiotics. In addition, antibacterial peptides can also be conjugated with antibiotics, lipids, or metal ions to prepare antibacterial peptides with special activities. This review introduces several methods for modifying peptide antibiotics and their specific applications, providing a theoretical basis for the further application of peptide antibiotics.

d-Amino Acid-Based Antifouling Peptides for the Construction of Electrochemical Biosensors Capable of Assaying Proteins in Serum with Enhanced Stability

The susceptibility of peptides to proteolytic degradation in human serum significantly hindered the potential application of peptide-based antifouling biosensors for long-term assaying of clinical samples. Herein, a robust antifouling biosensor with enhanced stability was constructed based on peptides composed of d-amino acids (d-peptide) with prominent proteolytic resistance. The electrode was electropolymerized with poly(3,4-ehtylenedioxythiophene) and electrodeposited with Au nanoparticles (AuNPs), and the d-peptide was then immobilized onto the AuNPs, and a typical antibody specific for immunoglobulin M (IgM) was immobilized. Because of the effect of d-amino acids, the d-peptide-modified electrode surface showed prominent antifouling capability and high tolerance to enzymatic hydrolysis. Moreover, the d-peptide-modified electrode exhibited much stronger long-term stability, as well as antifouling ability in human serum than the electrode modified with normal peptides. The electrochemical biosensor exhibited a sensitive response to IgM linearly within the range of 100 pg mL^-1 to 1.0 μg mL^-1 and a very low detection limit down to 37 pg mL^-1, and it was able to detect IgM in human serum with good accuracy. This work provided a new strategy to develop robust peptide-based biosensors to resist the proteolytic degradation for practical application in complex clinical samples.

Strategies employed in the design of antimicrobial peptides with enhanced proteolytic stability

Due to the alarming developing rate of multidrug-resistant bacterial pathogens, the development and modification of antimicrobial peptides (AMPs) are unprecedentedly active. Despite the fact that considerable efforts have been expended on the discovery and design strategies of AMPs, the clinical translation of peptide antibiotics remains inadequate. A large number of articles and reviews credited the limited success of AMPs to their poor stability in the biological environment, particularly their poor proteolytic stability. In the past forty years, various design strategies have been used to improve the proteolytic stability of AMPs, such as sequence modification, cyclization, peptidomimetics, and nanotechnology. Herein, we focus our discussion on the progress made in improving the proteolytic stability of AMPs and the principle, successes, and limitations of various anti-proteolytic design strategies. It is of prospective significance to extend current insights into the degradation-related inactivation of AMPs and also alleviate/overcome the problem.

Small Amphiphilic Peptides: Activity Against a Broad Range of Drug-Resistant Bacteria and Structural Insight into Membranolytic Properties

We report the synthesis and antibacterial activities of a series of amphiphilic membrane-active peptides composed, in part, of various nongenetically coded hydrophobic amino acids. The lead cyclic peptides, 8C and 9C, showed broad-spectrum activity against drug-resistant Gram-positive (minimum inhibitory concentration (MIC) = 1.5-6.2 μg/mL) and Gram-negative (MIC = 12.5-25 μg/mL) bacteria. The cytotoxicity study showed the predominant lethal action of the peptides against bacteria as compared with mammalian cells. A plasma stability study revealed approximately 2-fold higher stability of lead cyclic peptides as compared to their linear counterparts after 24 h of incubation. A calcein dye leakage experiment revealed the membranolytic effect of the cyclic peptides. Nuclear magnetic resonance spectroscopy and molecular dynamics simulation studies of the interaction of the peptides with the phospholipid bilayer provided a solid structural basis to explain the membranolytic action of the peptides with atomistic details. These results highlight the potential of newly designed amphiphilic peptides as the next generation of peptide-based antibiotics.

Advances on chemically modified antimicrobial peptides for generating peptide antibiotics

Antimicrobial peptides (AMPs) are pinpointed as promising molecules against antibiotic-resistant bacterial infections. Nevertheless, there is a discrepancy between the AMP sequences generated and the tangible outcomes in clinical trials. AMPs' limitations include enzymatic degradation, chemical/physical instability and toxicity toward healthy human cells. These factors compromise AMPs' bioavailability, resulting in limited therapeutic potential. To overcome such obstacles, peptidomimetic approaches, including glycosylation, PEGylation, lipidation, cyclization, grafting, D-amino acid insertion, stapling and dendrimers are promising strategies to fine-tune AMPs. Here we focused on chemical modifications applied for AMP optimization and how they have helped these peptide-based antibiotic candidates' design and translational potential.

OMN6 a novel bioengineered peptide for the treatment of multidrug resistant Gram negative bacteria

New antimicrobial agents are urgently needed, especially to eliminate multidrug resistant Gram-negative bacteria that stand for most antibiotic-resistant threats. In the following study, we present superior properties of an engineered antimicrobial peptide, OMN6, a 40-amino acid cyclic peptide based on Cecropin A, that presents high efficacy against Gram-negative bacteria with a bactericidal mechanism of action. The target of OMN6 is assumed to be the bacterial membrane in contrast to small molecule-based agents which bind to a specific enzyme or bacterial site. Moreover, OMN6 mechanism of action is effective on Acinetobacter baumannii laboratory strains and clinical isolates, regardless of the bacteria genotype or resistance-phenotype, thus, is by orders-of-magnitude, less likely for mutation-driven development of resistance, recrudescence, or tolerance. OMN6 displays an increase in stability and a significant decrease in proteolytic degradation with full safety margin on erythrocytes and HEK293T cells. Taken together, these results strongly suggest that OMN6 is an efficient, stable, and non-toxic novel antimicrobial agent with the potential to become a therapy for humans.

Antimicrobial and Anticancer Properties of Synthetic Peptides Derived from the Wasp Parachartergus fraternus

Agelaia-MPI and protonectin are antimicrobial peptides isolated from the wasp Parachartergus fraternus that show antimicrobial and neuroactive activities. Previously, two analogues of these peptides, neuroVAL and protonectin-F, were designed to reduce nonspecific toxicity and improve potency. Here, the three-dimensional structures of neuroVAL, protonectin and protonectin-F were determined by using circular dichroism and NMR spectroscopy. Antibacterial, antifungal, cytotoxic and hemolytic activities were tested for the parent peptides and analogues. All peptides showed moderate antimicrobial activity against Gram-positive bacteria, with agelaia-MPI being the most active. Protonectin and protonectin-F were found to be toxic to cancerous and noncancerous cell lines. Internalization experiments revealed that these peptides accumulate inside both cell types. By contrast, neuroVAL was nontoxic to all tested cells and was able to enter cells without accumulating. In summary, neuroVAL has potential as a nontoxic cell-penetrating peptide, while protonectin-F needs further modification to realize its potential as an antitumor peptide.

Secondary Structural Transformation of Bovine Lactoferricin Affects Its Antibacterial Activity

Lactoferricin (Lfcin) is a potent antibacterial peptide derived from lactoferrin by pepsin hydrolysis. It was hypothesized that structural transformation of Lfcin could affect its antibacterial function through forming and breaking of intramolecular disulfide bond. To prove this hypothesis, bovine Lfcin (bLfcin) and its two derivatives, bLfcin with a disulfide bond (bLfcin DB) and bLfcin with a mutation C36G (bLfcin C36G), were synthesized, purified, and identified. The circular dichroism (CD) spectra of the peptides were detected in solutions with different ionic and hydrophobic strength. Then, the secondary structure contents of the peptides were calculated on the basis of the CD spectra. The antibacterial activity of the peptides against Escherichia coli ATCC 25922, Salmonella typhimurium ATCC 14028, Shigella flexneri ATCC 12022, and Staphylococcus aureus ATCC 25923 was evaluated. The results showed that bLfcin and bLfcin C36G had similar percentages of secondary structure in water, while bLfcin and bLfcin DB had similar ratios of secondary structure under less hydrophobic conditions. The synthetic peptides exhibited antibacterial activity against all the tested bacteria, except for S. aureus ATCC 25923. bLfcin demonstrated higher antibacterial activity compared with its derivatives. The results suggested that bLfcin could transform its structure under alterative ionic strengths and hydrophobic conditions, and the transformation of structures was beneficial to enhancing the antibacterial function.

A Review on the Use of Antimicrobial Peptides to Combat Porcine Viruses

Viral infectious diseases pose a serious threat to animal husbandry, especially in the pig industry. With the rapid, continuous variation of viruses, a series of therapeutic measures, including vaccines, have quickly lost their efficacy, leading to great losses for animal husbandry. Therefore, it is urgent to find new drugs with more stable and effective antiviral activity. Recently, it has been reported that antimicrobial peptides (AMPs) have great potential for development and application in animal husbandry because of their significant antibacterial and antiviral activity, and the antiviral ability of AMPs has become a research hotspot. This article aims to review the research situation of AMPs used to combat viruses in swine production of animal husbandry, clarify the mechanism of action of AMPs on viruses and raise some questions, and explore the future potential of AMPs in animal husbandry.

Animal toxins - Nature's evolutionary-refined toolkit for basic research and drug discovery

Venomous animals have evolved toxins that interfere with specific components of their victim's core physiological systems, thereby causing biological dysfunction that aids in prey capture, defense against predators, or other roles such as intraspecific competition. Many animal lineages evolved venom systems independently, highlighting the success of this strategy. Over the course of evolution, toxins with exceptional specificity and high potency for their intended molecular targets have prevailed, making venoms an invaluable and almost inexhaustible source of bioactive molecules, some of which have found use as pharmacological tools, human therapeutics, and bioinsecticides. Current biomedically-focused research on venoms is directed towards their use in delineating the physiological role of toxin molecular targets such as ion channels and receptors, studying or treating human diseases, targeting vectors of human diseases, and treating microbial and parasitic infections. We provide examples of each of these areas of venom research, highlighting the potential that venom molecules hold for basic research and drug development.

ABC Exporters in Pathogenesis: Role of Synthetic Anti-Microbial Peptides

ABC exporters are involved in diverse cellular processes including lipid trafficking, drug resistance, pathogenesis etc. The greatest thrust has been in the area of drug resistance that explains the underlying well-crafted canonical architecture of its structure. Interestingly, ranging from structural organisation to subsequent design and delivery aspects lays the niche of antimicrobial peptides. One of the major highlight of this paper is the role of synthetic antimicrobial peptides in current scenario.

Cyclic gomesin, a stable redesigned spider peptide able to enter cancer cells

Anticancer chemo- and targeted therapies are limited in some cases due to strong side effects and/or drug resistance. Peptides have received renascent interest as anticancer therapeutics and are currently being considered as alternatives and/or as complementary to biologics and small-molecule drugs. Gomesin, a disulfide-rich host defense peptide expressed in the Brazilian spider Acanthoscurria gomesiana selectively targets and disrupts cancer cell membranes. In the current study, we employed a range of biophysical methodologies with model membranes and bioassays to investigate the use of a cyclic analogue of gomesin as a drug scaffold to internalize cancer cells. We found that cyclic gomesin can internalize cancer cells via endocytosis and direct membrane permeation. In addition, we designed an improved non-disruptive and non-toxic cyclic gomesin analogue by incorporating D-amino acids within the scaffold. This improved analogue retained the ability to enter cancer cells and can be used as a scaffold to deliver drugs. Efforts to investigate the internalization mechanism used by host defense peptides, and to improve their stability, potency, selectivity and ability to permeate cancer cell membranes will increase the opportunities to repurpose peptides as templates for designing alternative anticancer therapeutic leads.

Discovery and mechanistic studies of cytotoxic cyclotides from the medicinal herb Hybanthus enneaspermus

Cyclotides are plant-derived peptides characterized by an ∼30-amino acid-long cyclic backbone and a cystine knot motif. Cyclotides have diverse bioactivities, and their cytotoxicity has attracted significant attention for its potential anticancer applications. Hybanthus enneaspermus (Linn) F. Muell is a medicinal herb widely used in India as a libido enhancer, and a previous study has reported that it may contain cyclotides. In the current study, we isolated 11 novel cyclotides and 1 known cyclotide (cycloviolacin O2) from H. enneaspermus and used tandem MS to determine their amino acid sequences. We found that among these cyclotides, hyen C comprises a unique sequence in loops 1, 2, 3, 4, and 6 compared with known cyclotides. The most abundant cyclotide in this plant, hyen D, had anticancer activity comparable to that of cycloviolacin O2, one of the most cytotoxic known cyclotides. We also provide mechanistic insights into how these novel cyclotides interact with and permeabilize cell membranes. Results from surface plasmon resonance experiments revealed that hyen D, E, L, and M and cycloviolacin O2 preferentially interact with model lipid membranes that contain phospholipids with phosphatidyl-ethanolamine headgroups. The results of a lactate dehydrogenase assay indicated that exposure to these cyclotides compromises cell membrane integrity. Using live-cell imaging, we show that hyen D induces rapid membrane blebbing and cell necrosis. Cyclotide-membrane interactions correlated with the observed cytotoxicity, suggesting that membrane permeabilization and disintegration underpin cyclotide cytotoxicity. These findings broaden our knowledge on the indigenous Indian herb H. enneaspermus and have uncovered cyclotides with potential anticancer activity.

The Strategies of Pathogen-Oriented Therapy on Circumventing Antimicrobial Resistance

The emerging antimicrobial resistance (AMR) poses serious threats to the global public health. Conventional antibiotics have been eclipsed in combating with drug-resistant bacteria. Moreover, the developing and deploying of novel antimicrobial drugs have trudged, as few new antibiotics are being developed over time and even fewer of them can hit the market. Alternative therapeutic strategies to resolve the AMR crisis are urgently required. Pathogen-oriented therapy (POT) springs up as a promising approach in circumventing antibiotic resistance. The tactic underling POT is applying antibacterial compounds or materials directly to infected regions to treat specific bacteria species or strains with goals of improving the drug efficacy and reducing nontargeting and the development of drug resistance. This review exemplifies recent trends in the development of POTs for circumventing AMR, including the adoption of antibiotic-antibiotic conjugates, antimicrobial peptides, therapeutic monoclonal antibodies, nanotechnologies, CRISPR-Cas systems, and microbiota modulations. Employing these alternative approaches alone or in combination shows promising advantages for addressing the growing clinical embarrassment of antibiotics in fighting drug-resistant bacteria.

Cyclic Analogues of Horseshoe Crab Peptide Tachyplesin I with Anticancer and Cell Penetrating Properties

Tachyplesin-I (TI) is a host defense peptide from the horseshoe crab Tachypleus tridentatus that has outstanding potential as an anticancer therapeutic lead. Backbone cyclized TI (cTI) has similar anticancer properties to TI but has higher stability and lower hemolytic activity. We designed and synthesized cTI analogues to further improve anticancer potential and investigated structure-activity relationships based on peptide-membrane interactions, cellular uptake, and anticancer activity. The membrane-binding affinity and cytotoxic activity of cTI were found to be highly dependent on peptide hydrophobicity and charge. We describe two analogues with increased selectivity toward melanoma cells and one analogue with the ability to enter cells with high efficacy and low toxicity. Overall, the structure-activity relationship study shows that cTI can be developed as a membrane-active antimelanoma lead, or be employed as a cell penetrating peptide scaffold that can target and enter cells without damaging their integrity.

Characterization of Tachyplesin Peptides and Their Cyclized Analogues to Improve Antimicrobial and Anticancer Properties

Tachyplesin I, II and III are host defense peptides from horseshoe crab species with antimicrobial and anticancer activities. They have an amphipathic β-hairpin structure, are highly positively-charged and differ by only one or two amino acid residues. In this study, we compared the structure and activity of the three tachyplesin peptides alongside their backbone cyclized analogues. We assessed the peptide structures using nuclear magnetic resonance (NMR) spectroscopy, then compared the activity against bacteria (both in the planktonic and biofilm forms) and a panel of cancerous cells. The importance of peptide-lipid interactions was examined using surface plasmon resonance and fluorescence spectroscopy methodologies. Our studies showed that tachyplesin peptides and their cyclic analogues were most potent against Gram-negative bacteria and melanoma cell lines, and showed a preference for binding to negatively-charged lipid membranes. Backbone cyclization did not improve potency, but improved peptide stability in human serum and reduced toxicity toward human red blood cells. Peptide-lipid binding affinity, orientation within the membrane, and ability to disrupt lipid bilayers differed between the cyclized peptide and the parent counterpart. We show that tachyplesin peptides and cyclized analogues have similarly potent antimicrobial and anticancer properties, but that backbone cyclization improves their stability and therapeutic potential.

Short Cationic Peptide Derived from Archaea with Dual Antibacterial Properties and Anti-Infective Potential

Bacterial biofilms and associated infections represent one of the biggest challenges in the clinic, and as an alternative to counter bacterial infections, antimicrobial peptides have attracted great attention in the past decade. Here, ten short cationic antimicrobial peptides were generated through a sliding-window strategy on the basis of the 19-amino acid residue peptide, derived from a Pyrobaculum aerophilum ribosomal protein. PaDBS1R6F10 exhibited anti-infective potential as it decreased the bacterial burden in murine Pseudomonas aeruginosa cutaneous infections by more than 1000-fold. Adverse cytotoxic and hemolytic effects were not detected against mammalian cells. The peptide demonstrated structural plasticity in terms of its secondary structure in the different environments tested. PaDBS1R6F10 represents a promising antimicrobial agent against bacteria infections, without harming human cells.

Redesigning Arenicin-1, an Antimicrobial Peptide from the Marine Polychaeta Arenicola marina, by Strand Rearrangement or Branching, Substitution of Specific Residues, and Backbone Linearization or Cyclization

Arenicin-1, a β-sheet antimicrobial peptide isolated from the marine polychaeta Arenicola marina coelomocytes, has a potent, broad-spectrum microbicidal activity and also shows significant toxicity towards mammalian cells. Several variants were rationally designed to elucidate the role of structural features such as cyclization, a certain symmetry of the residue arrangement, or the presence of specific residues in the sequence, in its membranolytic activity and the consequent effect on microbicidal efficacy and toxicity. The effect of variations on the structure was probed using molecular dynamics simulations, which indicated a significant stability of the β-hairpin scaffold and showed that modifying residue symmetry and β-strand arrangement affected both the twist and the kink present in the native structure. In vitro assays against a panel of Gram-negative and Gram-positive bacteria, including drug-resistant clinical isolates, showed that inversion of the residue arrangement improved the activity against Gram-negative strains but decreased it towards Gram-positive ones. Variants with increased symmetry were somewhat less active, whereas both backbone-cyclized and linear versions of the peptides, as well as variants with R→K and W→F replacement, showed antimicrobial activity comparable with that of the native peptide. All these variants permeabilized both the outer and the inner membranes of Escherichia coli, suggesting that a membranolytic mechanism of action was maintained. Our results indicate that the arenicin scaffold can support a considerable degree of variation while maintaining useful biological properties and can thus serve as a template for the elaboration of novel anti-infective agents.

A C-Terminal Fragment of Chlorotoxin Retains Bioactivity and Inhibits Cell Migration

Chlorotoxin was originally isolated from the venom of the Israeli scorpion Leiurus quinquestriatus, and has potential as a tumor imaging agent based on its selective binding to tumor cells. Several targets have been suggested for chlorotoxin including voltage-gated chloride channels, and it has been shown to have anti-angiogenic activity and inhibit cell migration. The structure of chlorotoxin is stabilized by four disulfide bonds and contains β-sheet and helical structure. Interestingly, the reduced form has previously been shown to inhibit cell migration to the same extent as the wild type, but structural analysis indicates that the reduced form of the peptide does not maintain the native secondary structure and appears unstructured in solution. This lack of structure suggests that a short stretch of amino acids might be responsible for the bioactivity. To explore this hypothesis, we have synthesized fragments of chlorotoxin without disulfide bonds. As expected for such small peptides, NMR analysis indicated that the peptides were unstructured in solution. However, the peptide corresponding to the eight C-terminal residues inhibited cell migration, in contrast to the other fragments. Our results suggest that the C-terminal region plays a critical role in the bioactivity of chlorotoxin.

In Vitro and In Planta Cyclization of Target Peptides Using an Asparaginyl Endopeptidase from Oldenlandia affinis

Cyclization of the peptide backbone by connecting the N- and C-terminus can endow target peptides with favorable properties, such as increased stability or potential oral bioavailability. However, there are few tools available for carrying out this modification. Asparaginyl endopeptidases (AEPs) are a class of enzymes that typically work as proteases, but a subset is highly efficient at cyclization of the peptide backbone. In this chapter we describe how to utilize a cyclizing AEP (OaAEP1_b) to produce backbone-cyclized peptides both in planta and in vitro. Using the in planta method, OaAEP1_b and the target precursor peptide are coexpressed in the leaves of the model plant Nicotiana benthamiana, and cyclization of the target peptide occurs in planta. Using the in vitro method, purified recombinant OaAEP1_b produced in bacteria is used to cyclize the target precursor peptide in vitro.

Gomesin inhibits melanoma growth by manipulating key signaling cascades that control cell death and proliferation

Consistent with their diverse pharmacology, peptides derived from venomous animals have been developed as drugs to treat disorders as diverse as hypertension, diabetes and chronic pain. Melanoma has a poor prognosis due in part to its metastatic capacity, warranting further development of novel targeted therapies. This prompted us to examine the anti-melanoma activity of the spider peptides gomesin (AgGom) and a gomesin-like homolog (HiGom). AgGom and HiGom dose-dependently reduced the viability and proliferation of melanoma cells whereas it had no deleterious effects on non-transformed neonatal foreskin fibroblasts. Concordantly, gomesin-treated melanoma cells showed a reduced G0/G1 cell population. AgGom and HiGom compromised proliferation of melanoma cells via activation of the p53/p21 cell cycle check-point axis and the Hippo signaling cascade, together with attenuation of the MAP kinase pathway. We show that both gomesin peptides exhibit antitumoral activity in melanoma AVATAR-zebrafish xenograft tumors and that HiGom also reduces tumour progression in a melanoma xenograft mouse model. Taken together, our data highlight the potential of gomesin for development as a novel melanoma-targeted therapy.

Defense Peptides Engineered from Human Platelet Factor 4 Kill Plasmodium by Selective Membrane Disruption

Malaria is a serious threat to human health and additional classes of antimalarial drugs are greatly needed. The human defense protein, platelet factor 4 (PF4), has intrinsic antiplasmodial activity but also undesirable chemokine properties. We engineered a peptide containing the isolated PF4 antiplasmodial domain, which through cyclization, retained the critical structure of the parent protein. The peptide, cPF4PD, killed cultured blood-stage Plasmodium falciparum with low micromolar potency by specific disruption of the parasite digestive vacuole. Its mechanism of action involved selective penetration and accumulation inside the intraerythrocytic parasite without damaging the host cell or parasite membranes; it did not accumulate in uninfected cells. This selective activity was accounted for by observations of the peptide's specific binding and penetration of membranes with exposed negatively charged phospholipid headgroups. Our findings highlight the tremendous potential of the cPF4PD scaffold for developing antimalarial peptide drugs with a distinct and selective mechanism of action.

The Biological and Biophysical Properties of the Spider Peptide Gomesin

This review summarises the current knowledge of Gomesin (Gm), an 18-residue long, cationic anti-microbial peptide originally isolated from the haemocytes of the Brazilian tarantula Acanthoscurria gomesiana. The peptide shows potent cytotoxic activity against clinically relevant microbes including Gram-positive and Gram-negative bacteria, fungi, and parasites. In addition, Gm shows in-vitro and in-vivo anti-cancer activities against several human and murine cancers. The peptide exerts its cytotoxic activity by permeabilising cell membranes, but the underlying molecular mechanism of action is still unclear. Due to its potential as a therapeutic agent, the structure and membrane-binding properties, as well as the leakage and cytotoxic activities of Gm have been studied using a range of techniques. This review provides a summary of these studies, with a particular focus on biophysical characterisation studies of peptide variants that have attempted to establish a structure-activity relationship. Future studies are still needed to rationalise the binding affinity and cell-type-specific selectivity of Gm and its variants, while more pre-clinical studies are required to develop Gm into a therapeutically useful peptide.

Antimicrobial peptides with selective antitumor mechanisms: prospect for anticancer applications

In the last several decades, there have been significant advances in anticancer therapy. However, the development of resistance to cancer drugs and the lack of specificity related to actively dividing cells leading to toxic side effects have undermined these achievements. As a result, there is considerable interest in alternative drugs with novel antitumor mechanisms. In addition to the recent approach using immunotherapy, an effective but much cheaper therapeutic option of pharmaceutical drugs would still provide the best choice for cancer patients as the first line treatment. Ribosomally synthesized cationic antimicrobial peptides (AMPs) or host defense peptides (HDP) display broad-spectrum activity against bacteria based on electrostatic interactions with negatively charged lipids on the bacterial surface. Because of increased proportions of phosphatidylserine (negatively charged) on the surface of cancer cells compared to normal cells, cationic amphipathic peptides could be an effective source of anticancer agents that are both selective and refractory to current resistance mechanisms. We reviewed herein the prospect for AMP application to cancer treatment, with a focus on modes of action of cationic AMPs.

Designing improved active peptides for therapeutic approaches against infectious diseases

Infectious diseases are one of the main causes of human morbidity and mortality. In the last few decades, pathogenic microorganisms' resistance to conventional drugs has been increasing, and it is now pinpointed as a major worldwide health concern. The need to search for new therapeutic options, as well as improved treatment outcomes, has therefore increased significantly, with biologically active peptides representing a new alternative. A substantial research effort is being dedicated towards their development, especially due to improved biocompatibility and target selectivity. However, the inherent limitations of peptide drugs are restricting their application. In this review, we summarize the current status of peptide drug development, focusing on antiviral and antimicrobial peptide activities, highlighting the design improvements needed, and those already being used, to overcome the drawbacks of the therapeutic application of biologically active peptides.

Redesigned Spider Peptide with Improved Antimicrobial and Anticancer Properties

Gomesin, a disulfide-rich antimicrobial peptide produced by the Brazilian spider Acanthoscurria gomesiana, has been shown to be potent against Gram-negative bacteria and to possess selective anticancer properties against melanoma cells. In a recent study, a backbone cyclized analogue of gomesin was shown to be as active but more stable than its native form. In the current study, we were interested in improving the antimicrobial properties of the cyclic gomesin, understanding its selectivity toward melanoma cells and elucidating its antimicrobial and anticancer mode of action. Rationally designed analogues of cyclic gomesin were examined for their antimicrobial potency, selectivity toward cancer cells, membrane-binding affinity, and ability to disrupt cell and model membranes. We improved the activity of cyclic gomesin by ∼10-fold against tested Gram-negative and Gram-positive bacteria without increasing toxicity to human red blood cells. In addition, we showed that gomesin and its analogues are more toxic toward melanoma and leukemia cells than toward red blood cells and act by selectively targeting and disrupting cancer cell membranes. Preference toward some cancer types is likely dependent on their different cell membrane properties. Our findings highlight the potential of peptides as antimicrobial and anticancer leads and the importance of selectively targeting cancer cell membranes for drug development.

Potential role of an antimicrobial peptide, KLK in inhibiting lipopolysaccharide-induced macrophage inflammation

Antimicrobial peptides (AMPs) are attractive alternatives to antibiotics. Due to their immune modulatory properties, AMPs are at present emerging as promising agents for controlling inflammatory-mediated diseases. In this study, anti-inflammatory potential of an antimicrobial peptide, KLK (KLKLLLLLKLK) and its analogs was evaluated in lipopolysaccharide (LPS)-induced RAW 264.7 macrophages. The results herein demonstrated that KLK peptide as well as its analogs significantly inhibited the pro-inflammatory mediator nitric oxide (NO), interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) production in LPS-stimulated RAW 264.7 macrophages in dose-dependent manners, and such inhibitory effects were not due to direct cytotoxicity. When considering inhibition potency, KLK among the test peptides exhibited the most effective activity. The inhibitory activity of KLK peptide also extended to include suppression of LPS-induced production of prostaglandin E2 (PGE2). KLK significantly decreased mRNA and protein expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) as well as mRNA expression of IL-1β and TNF-α. Moreover, KLK inhibited nuclear translocation of nuclear factor-κB (NF-κB) p65 and blocked degradation and phosphorylation of inhibitor of κB (IκB). Taken together, these results suggested that the KLK peptide inhibited inflammatory response through the down-regulation of NF-κB mediated activation in macrophages. Since peptide analogs with different amino acid sequences and arrangement were investigated for their anti-inflammatory activities, the residues/structures required for activity were also discussed. Our findings therefore proved anti-inflammatory potential of the KLK peptide and provide direct evidence for therapeutic application of KLK as a novel anti-inflammatory agent.

Insights into Antimicrobial Peptides from Spiders and Scorpions

The venoms of spiders and scorpions contain a variety of chemical compounds. Antimicrobial peptides (AMPs) from these organisms were first discovered in the 1990s. As of May 2015, there were 42 spider's and 63 scorpion's AMPs in the Antimicrobial Peptide Database (http://aps.unmc.edu/AP). These peptides have demonstrated broad or narrow-spectrum activities against bacteria, fungi, viruses, and parasites. In addition, they can be toxic to cancer cells, insects and erythrocytes. To provide insight into such an activity spectrum, this article discusses the discovery, classification, structure and activity relationships, bioinformatics analysis, and potential applications of spider and scorpion AMPs. Our analysis reveals that, in the case of linear peptides, spiders use both glycine-rich and helical peptide models for defense, whereas scorpions use two distinct helical peptide models with different amino acid compositions to exert the observed antimicrobial activities and hemolytic toxicity. Our structural bioinformatics study improves the knowledge in the field and can be used to design more selective peptides to combat tumors, parasites, and viruses.

Structure-activity relationships and action mechanisms of collagen-like antimicrobial peptides

An antimicrobial triple-helical peptide, R3, was previously obtained from a collagen-like combinatorial peptide library. In this research, based on structure-activity relationship studies of R3, a more potent peptide, RR4, with increased positive net charge and charge density relative to R3, was developed. RR4 exhibited antimicrobial activity against both Gram-negative and Gram-positive bacterial strains, including multidrug-resistant strains. Its action could be attributed to entry into cells and interactions with intercellular molecules such as DNA/RNA that inhibited cell division rather than increasing bacterial membrane permeability. Furthermore, RR4 exhibited remarkable stability in serum and low cytotoxicity.

Design and characterization of short hybrid antimicrobial peptides from pEM-2, mastoparan-VT1, and mastoparan-B

Antimicrobial peptides are considered to be excellent templates for designing novel antibiotics because of their broad-spectrum antimicrobial activity and their low prognostic to induce antibiotic resistance. In this study, for the first time, a series of short hybrid antimicrobial peptides combined by different fragments of venom-derived alpha-helical antimicrobial peptides pEM-2, mastoparan-VT1, and mastoparan-B were designed with the intent to improve the therapeutic index of the parental peptides. Short hybrid antimicrobial peptides PV, derived from pEM-2 and mastoparan-VT1, was found to possess the highest antibacterial, hemolytic, and cytotoxic activity. Short hybrid antimicrobial peptides PV3, derived from pEM-2 and three fragments of mastoparan-VT1, showed more than threefold improvement in therapeutic index compared with parental peptides pEM-2 and mastoparan-VT1. PV had the highest antimicrobial activity in stability studies. Except BVP, designed based on all three parental peptides, the other short hybrid antimicrobial peptides at their minimal inhibitory concentration and 2× minimal inhibitory concentration required less than 120 and 60 min to reduce >3log10 the initial inoculum, respectively. All peptides had membrane-disrupting activity in a time-dependent manner. Collectively, this study highlights the potential for rational design of improved short hybrid antimicrobial peptides such as PV3 that was an ideal candidate for further assessment with the ultimate purpose of development of effective antimicrobial agents.

Cyclic Dipeptide Shuttles as a Novel Skin Penetration Enhancement Approach: Preliminary Evaluation with Diclofenac

This study demonstrates the effectiveness of a peptide shuttle in delivering diclofenac into and through human epidermis. Diclofenac was conjugated to a novel phenylalanyl-N-methyl-naphthalenylalanine-derived diketopiperazine (DKP) shuttle and to TAT (a classical cell penetrating peptide), and topically applied to human epidermis in vitro. DKP and TAT effectively permeated into and through human epidermis. When conjugated to diclofenac, both DKP and TAT enhanced delivery into and through human epidermis, though DKP was significantly more effective. Penetration of diclofenac through human epidermis (to receptor) was increased by conjugation to the peptide shuttle and cell penetrating peptide with enhancement of 6x by DKP-diclofenac and 3x by TAT-diclofenac. In addition, the amount of diclofenac retained within the epidermis was significantly increased by peptide conjugation. COX-2 inhibition activity of diclofenac was retained when conjugated to DKP. Our study suggests that the peptide shuttle approach may offer a new strategy for targeted delivery of small therapeutic and diagnostic molecules to the skin.

Defense peptides: recent developments

Defense peptides are small amphipathic molecules that exhibit antimicrobial, antitumor, antiviral, and immunomodulatory properties. This review summarizes current knowledge on the mechanisms of antimicrobial activity of cationic and anionic defense peptides, indicating peptide-based as well as microbial cell-based factors affecting this activity. The peptide-based factors include charge, hydrophibicity, and amphipathicity, whereas the pathogen-based factors are membrane lipid composition, presence of sterols, membrane fluidity, cell wall components, and secreted factors such as extracellular proteinases. Since defense peptides have been considered very promising molecules that could replace conventional antibiotics in the era of drug-resistant pathogens, the issue of microbial resistance to antimicrobial peptides (AMPs) is addressed. Furthermore, selected approaches employed for optimization and de novo design of effective AMPs based on the properties recognized as important for the function of natural defense peptides are presented.

Efficient backbone cyclization of linear peptides by a recombinant asparaginyl endopeptidase

Cyclotides are diverse plant backbone cyclized peptides that have attracted interest as pharmaceutical scaffolds, but fundamentals of their biosynthetic origin remain elusive. Backbone cyclization is a key enzyme-mediated step of cyclotide biosynthesis and confers a measure of stability on the resultant cyclotide. Furthermore, cyclization would be desirable for engineered peptides. Here we report the identification of four asparaginyl endopeptidases (AEPs), proteases implicated in cyclization, from the cyclotide-producing plant Oldenlandia affinis. We recombinantly express OaAEP1_b and find it functions preferably as a cyclase by coupling C-terminal cleavage of propeptide substrates with backbone cyclization. Interestingly, OaAEP1_b cannot cleave at the N-terminal site of O. affinis cyclotide precursors, implicating additional proteases in cyclotide biosynthesis. Finally, we demonstrate the broad utility of this enzyme by cyclization of peptides unrelated to cyclotides. We propose that recombinant OaAEP1_b is a powerful tool for use in peptide engineering applications where increased stability of peptide products is desired.

Cyclotides are plant backbone-cyclised peptides with potential as pharmaceutical scaffolds. Here the authors report on the efficient backbone cyclization of cyclotides and unrelated peptides by a newly identified asparaginyl endopeptidase from Oldenlandia affinis.