Bacterium-Derived Cell-Penetrating Peptides Deliver Gentamicin To Kill Intracellular Pathogens

Protease Stabilizing Antimicrobial Peptide D1018M Showed Potent Antibiofilm and Anti-Intracellular Bacteria Activity Against MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) poses a major threat to human health and food safety, especially when bacteria form biofilms or invade host cells, which may cause recurring infections. A new solution is therefore urgently needed. The antimicrobial peptide innate defense regulator (IDR)-1018 and its derived peptide 1018M showed promising antimicrobial and antibiofilm activities. Nevertheless, their antibacterial efficacy against intracellular MRSA and protease tolerance remains to be promoted. Therefore, we synthesized D-amino acid substitution peptides D1018 and D1018M. The antimicrobial activity against MRSA of these novel peptides was increased by 1-fold (D1018) or remained constant (D1018M) compared with L-amino acids peptides. Their bactericidal mechanisms involve cell wall destruction, membrane penetration, and genomic DNA disruption. As expected, the stability of D1018 and D1018M was increased by 2-32 times against pepsin, trypsin, and cathepsin K. In addition, by D-amino acids substitution, the antibiofilm ability of D1018 was increased by 1.6 times, and the anti-intracellular bacterial activity of D1018M was improved 3.2-5.7 orders of magnitude. These data indicated that D1018M is a potential antimicrobial candidate for recurring MRSA infections.

Nanogels conjugated with cell-penetrating peptide as drug delivery vehicle for treating urinary tract infections

Among hospital-acquired infections, Pseudomonas aeruginosa-associated urinary tract infections (UTIs) are mainly caused by indwelling urethral catheters (catheter-associated UTIs or CAUTIs) and are difficult to treat, resulting in high rates of morbidity among hospitalized patients. While antibiotics can successfully treat bacteria in the bladder lumen, they are inefficient at crossing stratified urothelium plasma membranes to kill persistent intracellular bacterial communities (IBCs). Herein, we introduce an approach to target UTI IBCs by locally delivering the antibiotic gentamicin via polymeric nanogels conjugated with a cell-penetrating peptide Cys-Gly-Lys-Arg-Lys. This novel approach delivered ~36 % more intracellular gentamicin compared to drug delivered in solution in vitro. In an acute UTI murine model, the nanogel cell-penetrating peptide drug delivery system facilitated the transport of gentamicin into the urothelium and resulted in >90 % clearance of a uropathogenic P. aeruginosa clinical strain in vivo.

Strategies for the design of biomimetic cell-penetrating peptides using AI-driven in silico tools for drug delivery

Cell-penetrating peptides (CPP) have gained rapid attention over the last 25 years; this is attributed to their versatility, customisation, and 'Trojan horse' delivery that evades the immune system. However, the current CPP rational design process is limited, as it requires several rounds of peptide synthesis, prediction and wet-lab validation, which is expensive, time-consuming and requires extensive knowledge in peptide chemistry. Artificial intelligence (AI) has emerged as a promising alternative which can augment the design process, for example by determining physiochemical characteristics, secondary structure, solvent accessibility, disorder and flexibility, as well as predicting in vivo behaviour such as toxicity and peptidase degradation. Other more recent tools utilise supervised machine learning (ML) to predict the penetrative ability of an amino acid sequence. The use of AI in the CPP design process has the potential to reduce development costs and increase the chances of success with respect to delivery. This review provides a survey of in silico tools and AI platforms which can be utilised in the design process, and the key features that should be taken into consideration when designing next generation CPPs.

Current developments and prospects of the antibiotic delivery systems

Antibiotics have remained the cornerstone for the treatment of bacterial infections ever since their discovery in the twentieth century. The uproar over antibiotic resistance among bacteria arising from genome plasticity and biofilm development has rendered current antibiotic therapies ineffective, urging the development of innovative therapeutic approaches. The development of antibiotic resistance among bacteria has further heightened the clinical failure of antibiotic therapy, which is often linked to its low bioavailability, side effects, and poor penetration and accumulation at the site of infection. In this review, we highlight the potential use of siderophores, antibodies, cell-penetrating peptides, antimicrobial peptides, bacteriophages, and nanoparticles to smuggle antibiotics across impermeable biological membranes to achieve therapeutically relevant concentrations of antibiotics and combat antimicrobial resistance (AMR). We will discuss the general mechanisms via which each delivery system functions and how it can be tailored to deliver antibiotics against the paradigm of mechanisms underlying antibiotic resistance.

Azithromycin inhibits the intracellular persistence of Acinetobacter baumannii by inducing host cell autophagy in human bronchial epithelial cells

The invasion of host cells by bacteria, leading to intracellular infections, is a major cause of infection recurrence. Drug-resistant Acinetobacter baumannii (A. baumannii) is one of the most challenging public health issues worldwide, with very limited clinical treatment options available. A. baumannii has been found to be able to invade host cells and proliferate within them in recent studies. In addition to the direct antimicrobial effect of antibiotics, the activation of host autophagic flux also plays an important role in eliminating intracellular pathogens. Herein, this study aimes to evaluate the clearance effect of antibiotics on intracellular A. baumannii both in vivo and in vitro, and explore the relationship between this effect and autophagy. The results showed that intracellular pathogens resulted in a significant increase in the minimum bactericidal concentration, while azithromycin can significantly eliminate intracellular A. baumannii in vitro and in vivo. Notably, 60 μg/mL azithromycin demonstrated intracellular clearance against multidrug-resistant A. baumannii and markedly induced autophagosomes in BEAS-2B cells with a mild stimulation of autophagosomes degradation. These findings indicated that azithromycin can significantly clear intracellular A. baumannii and its ability to clear intracellular A. baumannii may be related to the stimulation of autophagosome formation and the induction of host autophagy, which has important implications for the clinical treatment of A. baumannii infections, especially when intracellular infections are present.

Dual Antibiotic Approach: Synthesis and Antibacterial Activity of Antibiotic-Antimicrobial Peptide Conjugates

In recent years, bacterial resistance to conventional antibiotics has become a major concern in the medical field. The global misuse of antibiotics in clinics, personal use, and agriculture has accelerated this resistance, making infections increasingly difficult to treat and rendering new antibiotics ineffective more quickly. Finding new antibiotics is challenging due to the complexity of bacterial mechanisms, high costs and low financial incentives for the development of new molecular scaffolds, and stringent regulatory requirements. Additionally, innovation has slowed, with many new antibiotics being modifications of existing drugs rather than entirely new classes. Antimicrobial peptides (AMPs) are a valid alternative to small-molecule antibiotics offering several advantages, including broad-spectrum activity and a lower likelihood of inducing resistance due to their multifaceted mechanisms of action. However, AMPs face challenges such as stability issues in physiological conditions, potential toxicity to human cells, high production costs, and difficulties in large-scale manufacturing. A reliable strategy to overcome the drawbacks associated with the use of small-molecule antibiotics and AMPs is combination therapy, namely the simultaneous co-administration of two or more antibiotics or the synthesis of covalently linked conjugates. This review aims to provide a comprehensive overview of the literature on the development of antibiotic-AMP conjugates, with a particular emphasis on critically analyzing the design and synthetic strategies employed in their creation. In addition to the synthesis, the review will also explore the reported antibacterial activity of these conjugates and, where available, examine any data concerning their cytotoxicity.

TriplEP-CPP: Algorithm for Predicting the Properties of Peptide Sequences

Advancements in medicine and pharmacology have led to the development of systems that deliver biologically active molecules inside cells, increasing drug concentrations at target sites. This improves effectiveness and duration of action and reduces side effects on healthy tissues. Cell-penetrating peptides (CPPs) show promise in this area. While traditional medicinal chemistry methods have been used to develop CPPs, machine learning techniques can speed up and reduce costs in the search for new peptides. A predictive algorithm based on machine learning models was created to identify novel CPP sequences using molecular descriptors using a combination of algorithms like k-nearest neighbors, gradient boosting, and random forest. Some potential CPPs were found and tested for cytotoxicity and penetrating ability. A new low-toxicity CPP was discovered from the Rhopilema esculentum venom proteome through this study.

Intracellular bacterial eradication using a novel peptide in vitro

AIM

To determine the effect of a novel antimicrobial peptide (AMP; OP145) and cell-penetrating peptide (Octa-arginine/R8) conjugate on the killing of intracellular Enterococcus faecalis, compared to OP145 and an antibiotic combination recommended for regenerative endodontic procedures.

METHODOLOGY

The biocompatible concentrations of OP145 and OP145-R8 were determined by assessing their cytotoxicity against human macrophages and red blood cells. Spatiotemporal internalization of the peptides into macrophages was investigated qualitatively and quantitatively by confocal laser scanning microscopy and flow cytometry respectively. Killing of extracellular and intracellular E. faecalis OG1RF by the peptides was determined by counting the colony-forming units (CFU). Intracellular antibacterial activity of the peptides was compared to a double antibiotic combination. Confocal microscopy was used to confirm the intracellular bacterial eradication. Significant differences between the different test groups were analysed using one-way analysis of variance. p < .05 was considered to be statistically significant.

RESULTS

Peptides at a concentration of 7.5 μmol/L were chosen for subsequent experiments based on the results of the alamarBlue™ cell viability assay and haemolytic assay. OP145-R8 selectively internalized into lysosomal compartments and the cytosol of macrophages. Conjugation with R8 improved the internalization of OP145 into macrophages in a temporal manner (70.53% at 1 h to 77.13% at 2 h), while no temporal increase was observed for OP145 alone (60.53% at 1 h with no increase at 2 h). OP145-R8 demonstrated significantly greater extracellular and intracellular antibacterial activity compared to OP145 at all investigated time-points and concentrations (p < .05). OP145-R8 at 7.5 μmol/L eradicated intracellular E. faecalis after 2 h (3.5 log reduction compared to the control; p < .05), while the antibiotics could not reduce more than 0.5 log CFU compared to the control (p > .05). Confocal microscopy showed complete absence of E. faecalis within the OP145-R8 treated macrophages.

CONCLUSIONS

The results of this study demonstrated that the conjugation of an AMP OP145 to a cell-penetrating peptide R8 eradicated extracellular and intracellular E. faecalis OG1RF without toxic effects on the host cells.

Intracellular bacteria in cancer-prospects and debates

Recent evidence suggests that some human cancers may harbor low-biomass microbial ecosystems, spanning bacteria, viruses, and fungi. Bacteria, the most-studied kingdom in this context, are suggested by these studies to localize within cancer cells, immune cells and other tumor microenvironment cell types, where they are postulated to impact multiple cancer-related functions. Herein, we provide an overview of intratumoral bacteria, while focusing on intracellular bacteria, their suggested molecular activities, communication networks, host invasion and evasion strategies, and long-term colonization capacity. We highlight how the integration of sequencing-based and spatial techniques may enable the recognition of bacterial tumor niches. We discuss pitfalls, debates and challenges in decisively proving the existence and function of intratumoral microbes, while reaching a mechanistic elucidation of their impacts on tumor behavior and treatment responses. Together, a causative understanding of possible roles played by intracellular bacteria in cancer may enable their future utilization in diagnosis, patient stratification, and treatment.

Protein Transduction System Based on Tryptophan-zipper against Intracellular Infections via Inhibiting Ferroptosis of Macrophages

Cells penetrating molecules in living systems hold promise of capturing and eliminating threats and damage that can plan intracellular fate promptly. However, it remains challenging to construct cell penetration systems that are physiologically stable with predictable self-assembly behavior and well-defined mechanisms. In this study, we develop a core-shell nanoparticle using a hyaluronic acid (HA)-coated protein transduction domain (PTD) derived from the human immunodeficiency virus (HIV). This nanoparticle can encapsulate pathogens, transporting the PTD into macrophages via lipid rafts. PTD forms hydrogen bonds with the components of the membrane through TAT, which has a high density of positive charges and reduces the degree of membrane order through Tryptophan (Trp)-zipper binding to the acyl tails of phospholipid molecules. HA-encapsulated PTD increases the resistance to trypsin and proteinase K, thereby penetrating macrophages and eliminating intracellular infections. Interestingly, the nonagglutination mechanism of PTD against pathogens ensures the safe operation of the cellular system. Importantly, PTD can activate the critical pathway of antiferroptosis in macrophages against pathogen infection. The nanoparticles developed in this study demonstrate safety and efficacy against Gram-negative and Gram-positive pathogens in three animal models. Overall, this work highlights the effectiveness of the PTD nanoparticle in encapsulating pathogens and provides a paradigm for transduction systems-anti-intracellular infection therapy.

Antimicrobial peptides´ immune modulation role in intracellular bacterial infection

Intracellular bacteria cause a wide range of diseases, and their intracellular lifestyle makes infections difficult to resolve. Furthermore, standard therapy antibiotics are often unable to eliminate the infection because they have poor cellular uptake and do not reach the concentrations needed to kill bacteria. In this context, antimicrobial peptides (AMPs) are a promising therapeutic approach. AMPs are short cationic peptides. They are essential components of the innate immune response and important candidates for therapy due to their bactericidal properties and ability to modulate host immune responses. AMPs control infections through their diverse immunomodulatory effects stimulating and/or boosting immune responses. This review focuses on AMPs described to treat intracellular bacterial infections and the known immune mechanisms they influence.

Hydrophobic modification improves the delivery of cell-penetrating peptides to eliminate intracellular pathogens in animals

Infections induced by intracellular pathogens are difficult to eradicate due to poor penetration of antimicrobials into cell membranes. It is of great importance to develop a new generation of antibacterial agents with dual functions of efficient cell penetration and bacterial inhibition. In this study, the association between hydrophobicity and cell-penetrating peptide delivery efficiency was investigated by fragment interception and hydrophobicity modification of natural porcine antimicrobial peptide PR-39 and the combination of cationic cell-penetrating peptide (R6) with antimicrobial peptide fragments modified with hydrophobic residues. The chimeric peptides P3I7 and P3L7, obtained through biofunctional screening, exhibited potent broad-spectrum antibacterial activity and low cytotoxicity. Moreover, P3I7 and P3L7 can effectively penetrate cells to eliminate intracellular pathogens mainly through endocytosis. The membrane destruction mechanism makes the peptides fast sterilizers and less prone to developing drug resistance. Finally, their good biocompatibility and antibacterial infection effects were verified in mice and piglets. To conclude, the chimeric peptides P3I7 and P3L7 show great potential as affordable and effective antimicrobial agents and may serve as ideal candidates for the treatment of intracellular bacterial infections. STATEMENT OF SIGNIFICANCE: The low permeability of antibacterial drugs makes infections induced by intracellular bacteria extremely difficult to treat. To address this issue, we designed chimeric peptides with dual cell-penetrating and antibacterial functions. The active peptides P3I7 and P3L7, acquired through functional screening have strong broad-spectrum antibacterial activity and powerful bactericidal effects against intracellular Staphylococcus aureus. The membrane permeation mechanism of P3I7 and P3L7 against bacteria endows fast bactericidal activity with low drug resistance. The biosafety and antibacterial activity of P3I7 and P3L7 were also validated by in vivo trials. This study provides an ideal drug candidate against intracellular bacterial infections.

Antimicrobial and Cell-Penetrating Peptides: Understanding Penetration for the Design of Novel Conjugate Antibiotics

Antimicrobial peptides (AMPs) are short oligopeptides that can penetrate the bacterial inner and outer membranes. Together with cell-penetrating peptides (CPPs), they are called membrane active peptides; peptides which can translocate across biological membranes. Over the last fifty years, attempts have been made to understand the molecular features that drive the interactions of membranes with membrane active peptides. This review examines the features of a membrane these peptides exploit for translocation, as well as the physicochemical characteristics of membrane active peptides which are important for translocation. Moreover, it presents examples of how these features have been used in recent years to create conjugates consisting of a membrane active peptide, called a "vector", attached to either a current or novel antibiotic, called a "cargo" or "payload". In addition, the review discusses what properties may contribute to an ideal peptide vector able to deliver cargoes across the bacterial outer membrane as the rising issue of antimicrobial resistance demands new strategies to be employed to combat this global public health threat.

Strategies and progresses for enhancing targeted antibiotic delivery

Antibiotic resistance is a global health issue and a potential risk for society. Antibiotics administered through conventional formulations are devoid of targeting effect and often spread to various undesired body sites, leading to sub-lethal concentrations at the site of action and thus resulting in emergence of resistance, as well as side effects. Moreover, we have a very slim antibiotic pipeline. Drug-delivery systems have been designed to control the rate, time, and site of drug release, and innovative approaches for antibiotic delivery provide a glint of hope for addressing these issues. This review elaborates different delivery strategies and approaches employed to overcome the limitations of conventional antibiotic therapy. These include antibiotic conjugates, prodrugs, and nanocarriers for local and targeted antibiotic release. In addition, a wide range of stimuli-responsive nanocarriers and biological carriers for targeted antibiotic delivery are discussed. The potential advantages and limitations of targeted antibiotic delivery strategies are described along with possible solutions to avoid these limitations. A number of antibiotics successfully delivered through these approaches with attained outcomes and potentials are reviewed.

Intracellular Bacterial Targeting by a Thiazolyl Benzenesulfonamide and Octaarginine Peptide Complex

A brominated thiazolyl benzenesulfonamide (BTB) derivative is conjugated with the cell-penetrating peptide octaarginine (R8) in an effort to construct innovative antibacterial products. The noncovalent complex of BTB and R8 is characterized by Fourier transform infrared (FTIR) spectroscopy, which indicates hydrogen bonding between the two constituents. Attachment of the peptide moiety renders aqueous solubility to the hydrophobic benzenesulfonamide drug and bestows bactericidal activity. Confocal imaging in conjunction with dye probes shows successful clearance of intracellular Staphylococcus aureus bacteria by the BTB-R8 complex. Scanning electron micrographs and studies with a set of fluorescent dyes suggest active disruption of the bacterial cell membrane by the BTB-R8 complex. In contrast, the complex of BTB with octalysine (K8) fails to cause membrane damage and displays a modest antibacterial effect. A complex of BTB with the water-soluble hydrophilic polymer poly(vinylpyrrolidone) (PVP) does not display any antibacterial effect, indicating the distinctive role of the cell-penetrating peptide (CPP) R8 in the cognate complex. The leakage of the encapsulated dye from giant unilamellar vesicles upon interaction with the BTB-R8 complex further highlights the membrane activity of the complex, which cannot be accomplished by bare sulfonamide alone. This work broadens the scope of use of CPPs with respect to eliciting antibacterial activity and potentially expands the limited arsenal of membrane-targeting antibiotics.

Cell-penetrating peptides improve pharmacokinetics and pharmacodynamics of anticancer drugs

This review concentrates on the research concerning conjugates of anticancer drugs with versatile cell-penetrating peptides (CPPs). For a better insight into the relationship between the components of the constructs, it starts with the characteristic of the peptides and considers its following aspects: mechanisms of cellular internalization, interaction with cancer-modified membranes, selectivity against tumor tissue. Also, CPPs with anticancer activity have been distinguished and summarized with their mechanisms of action. With respect to the conjugates, the preclinical studies (in vitro, in vivo) indicated that they possess several merits in comparison to the parent drugs. They concerned not only better cellular internalization but also other improvements in pharmacokinetics (e.g. access to the brain tissue) and pharmacodynamics (e.g. overcoming drug resistance). The anticancer activity of the conjugates was usually superior to that of the unconjugated drug. Certain anticancer CPPs and conjugates entered clinical trials.

Delivery of Antibiotics by Cell-Penetrating Peptides to Kill Intracellular Pathogens

In the last decades, the increasing rate of multidrug-resistant bacteria to classical antibiotics has driven research towards identification of other means to fight bacterial infections. In this context, intracellular and/or invasive facultative intracellular bacteria represent a particular problem as common antimicrobials are often not able to reach an effective intracellular concentration. In this regard, cell-penetrating peptides (CPP) can mediate the internalization of previously nonpermeable antimicrobial compounds into the cytoplasm of host cells where they efficiently kill intracellular pathogens. This chapter describes the conjugation of CPPs with antimicrobial agents for the delivery into infected cells. Furthermore, different antimicrobial activity assays will be described including the CPP-mediated delivery of an antimicrobial agent for the treatment of intracellular infections.

Biotherapeutic effect of cell-penetrating peptides against microbial agents: a review

Selective permeability of biological membranes represents a significant barrier to the delivery of therapeutic substances into both microorganisms and mammalian cells, restricting the access of drugs into intracellular pathogens. Cell-penetrating peptides usually 5-30 amino acids with the characteristic ability to penetrate biological membranes have emerged as promising antimicrobial agents for treating infections as well as an effective delivery modality for biological conjugates such as nucleic acids, drugs, vaccines, nanoparticles, and therapeutic antibodies. However, several factors such as antimicrobial resistance and poor drug delivery of the existing medications justify the urgent need for developing a new class of antimicrobials. Herein, we review cell-penetrating peptides (CPPs) used to treat microbial infections. Although these peptides are biologically active for infections, effective transduction into membranes and cargo transport, serum stability, and half-life must be improved for optimum functions and development of next-generation antimicrobial agents.

Emergent antibacterial activity of N-(thiazol-2-yl)benzenesulfonamides in conjunction with cell-penetrating octaarginine

Hybrid antimicrobials that combine the effect of two or more agents represent a promising antibacterial therapeutic strategy. In this work, we have synthesized N-(4-(4-(methylsulfonyl)phenyl)-5-phenylthiazol-2-yl)benzenesulfonamide derivatives that combine thiazole and sulfonamide, groups with known antibacterial activity. These molecules are investigated for their antibacterial activity, in isolation and in complex with the cell-penetrating peptide octaarginine. Several of the synthesized compounds display potent antibacterial activity against both Gram-negative and Gram-positive bacteria. Compounds with 4-tert-butyl and 4-isopropyl substitutions exhibit attractive antibacterial activity against multiple strains. The isopropyl substituted derivative displays low MIC of 3.9 μg mL⁻¹ against S. aureus and A. xylosoxidans. The comparative antibacterial behaviour of drug–peptide complex, drug alone and peptide alone indicates a distinctive mode of action of the drug–peptide complex, that is not the simple sum total of its constituent components. Specificity of the drug–peptide complex is evident from comparison of antibacterial behaviour with a synthetic intermediate–peptide complex. The octaarginine–drug complex displays faster killing-kinetics towards bacterial cells, creates pores in the bacterial cell membranes and shows negligible haemolytic activity towards human RBCs. Our results demonstrate that mere attachment of a hydrophobic moiety to a cell penetrating peptide does not impart antibacterial activity to the resultant complex. Conversely, the work suggests distinctive modes of antibiotic activity of small molecules when used in conjunction with a cell penetrating peptide.

Hybrid antimicrobials that combine the effect of two or more agents represent a promising antibacterial therapeutic strategy.

Antibiotic-cell-penetrating peptide conjugates targeting challenging drug-resistant and intracellular pathogenic bacteria

The failure to treat everyday bacterial infections is a current threat as pathogens are finding new ways to thwart antibiotics through mechanisms of resistance and intracellular refuge, thus rendering current antibiotic strategies ineffective. Cell-penetrating peptides (CPPs) are providing a means to improve antibiotics that are already approved for use. Through coadministration and conjugation of antibiotics with CPPs, improved accumulation and selectivity with alternative and/or additional modes of action against infections have been observed. Herein, we review the recent progress of this antibiotic-cell-penetrating peptide strategy in combatting sensitive and drug-resistant pathogens. We take a closer look into the specific antibiotics that have been enhanced, and in some cases repurposed as broad-spectrum drugs. Through the addition and conjugation of cell-penetrating peptides to antibiotics, increased permeation across mammalian and/or bacterial membranes and a broader range in bacterial selectivity have been achieved.

Critical Side Chain Effects of Cell-Penetrating Peptides for Transporting Oligo Peptide Nucleic Acids in Bacteria

Of various methods for delivering functional molecules into cells, a chemical approach using cell-penetrating peptides (CPPs) is facile and highly efficient. Currently, however, there are few examples of CPPs highly efficient with bacteria in contrast to CPPs targeting animal cells, and thus our understanding of the structural effects of these bacteria-efficient CPPs, termed as BCPPs, on permeation efficiency is limited. Herein, we report a comprehensive investigation on the permeation efficiencies of cationic short peptides through bacterial cell membranes. We observed that elongating the length of the main chain increased permeation efficiency. More interestingly, the length of the peptide side chain critically affected permeation efficiency; shortening the side chain significantly enhanced efficiency. Among the BCPPs investigated, 2,3-diaminopropionic acid nonamer showed the highest permeation efficiency into bacterial cells of diverse strains, allowing the transport of oligo peptide nucleic acids and subsequent growth inhibition. This study provides insights into the molecular design of efficient BCPPs for manipulating bacterial growth.

Marine Peptide-N6NH2 and Its Derivative-GUON6NH2 Have Potent Antimicrobial Activity Against Intracellular Edwardsiella tarda in vitro and in vivo

Edwardsiella tarda is a facultative intracellular pathogen in humans and animals. There is no effective way except vaccine candidates to eradicate intracellular E. tarda. In this study, four derivatives of marine peptide-N6NH2 were designed by an introduction of unnatural residues or substitution of natural ones, and their intracellular activities against E. tarda were evaluated in macrophages and in mice, respectively. The minimum inhibitory concentration (MIC) value of N6NH2 and GUON6NH2 against E. tarda was 8 μg/mL. GUON6NH2 showed higher stability to trypsin, lower toxicity (<1%) and longer post-antibiotic effect (PAE) than N6NH2 and other derivatives. Antibacterial mechanism results showed that GUON6NH2 could bind to LPS and destroyed outer/inner cell membranes of E. tarda, superior to N6NH2 and norfloxacin. Both N6NH2 and GUON6NH2 were internalized into macrophages mainly via lipid rafts, micropinocytosis, and microtubule polymerization, respectively, and distributed in the cytoplasm. The intracellular inhibition rate of GUON6NH2 against E. tarda was 97.05–100%, higher than that in case of N6NH2 (96.82–100%). In the E. tarda-induced peritonitis mouse model, after treatment with of 1 μmol/kg N6NH2 and GUON6NH2, intracellular bacterial numbers were reduced by 1.54- and 1.97-Log₁₀ CFU, respectively, higher than norfloxacin (0.35-Log₁₀ CFU). These results suggest that GUON6NH2 may be an excellent candidate for novel antimicrobial agents to treat infectious diseases caused by intracellular E. tarda.

Validation of Suitable Carrier Molecules and Target Genes for Antisense Therapy Using Peptide-Coupled Peptide Nucleic Acids (PNAs) in Streptococci

Antisense peptide nucleic acids (PNAs) targeting genes involved in metabolism or virulence are a possible means to treat infections or to investigate pathogenic bacteria. Potential targets include essential genes, virulence factor genes, or antibiotic resistance genes. For efficient cellular uptake, PNAs can be coupled to cell-penetrating peptides (CPPs). CPPs are peptides that serve as molecular transporters and are characterized by a comparably low cytotoxicity. So far, there is only limited information about CPPs that mediate PNA uptake by Gram-positive bacteria. Here, we describe two methods to identify suitable CPP-antisense PNA conjugates, novel carrier molecules, and efficient target genes for streptococcal species and to evaluate their antimicrobial efficiency.

Potent intracellular antibacterial activity of a marine peptide-N6NH2 and its D-enantiomer against multidrug-resistant Aeromonas veronii

Aeromonas veronii can cause a variety of diseases such as sepsis in humans and animals. However, there has been no effective way to eradicate A. veronii. In this study, the intracellular antibacterial activities of the C-terminal aminated marine peptide N6 (N6NH₂) and its D-enantiomer (DN6NH₂) against A. veronii were investigated in macrophages and in mice, respectively. The result showed that DN6NH₂ with the minimum inhibitory concentration (MIC) of 1.62 μM is more resistant to cathepsin B than N6NH₂ (3.23 μM). The penetration percentages of the cells treated with 4-200 μg/mL fluorescein isothiocyanate (FITC)-DN6NH₂ were 52.5-99.6%, higher than those of FITC-N6NH₂ (27.0-99.1%). Both N6NH₂ and DN6NH₂ entered macrophages by macropinocytosis and an energy-dependent manner. DN6NH₂ reduced intracellular A. veronii by 34.57%, superior to N6NH₂ (19.52%). After treatment with 100 μg/mL DN6NH₂, the levels of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and IL-1β were reduced by 53.45%, 58.54%, and 44.62%, respectively, lower than those of N6NH₂ (15.65%, 12.88%, and 14.10%, respectively); DN6NH₂ increased the IL-10 level (42.94%), higher than N6NH₂ (7.67%). In the mice peritonitis model, 5 μmol/kg DN6NH₂ reduced intracellular A. veronii colonization by 73.22%, which was superior to N6NH₂ (32.45%) or ciprofloxacin (45.67%). This suggests that DN6NH₂ may be used as the candidate for treating intracellular multidrug-resistant (MDR) A. veronii. KEY POINTS: • DN6NH₂ improved intracellular antibacterial activity against MDR A. veronii. • DN6NH₂ entered macrophages by micropinocytosis and enhanced the internalization rates. • DN6NH₂ effectively protected the mice from infection with A. veronii.

Antimicrobial Peptides and Cell-Penetrating Peptides for Treating Intracellular Bacterial Infections

Bacterial infections caused by intracellular pathogens are difficult to control. Conventional antibiotic therapies are often ineffective, as high doses are needed to increase the number of antibiotics that will cross the host cell membrane to act on the intracellular bacterium. Moreover, higher doses of antibiotics may lead to elevated severe toxic effects against host cells. In this context, antimicrobial peptides (AMPs) and cell-penetrating peptides (CPPs) have shown great potential to treat such infections by acting directly on the intracellular pathogenic bacterium or performing the delivery of cargos with antibacterial activities. Therefore, in this mini-review, we cover the main AMPs and CPPs described to date, aiming at intracellular bacterial infection treatment. Moreover, we discuss some of the proposed mechanisms of action for these peptide classes and their conjugation with other antimicrobials.

"Metaphilic" Cell-Penetrating Polypeptide-Vancomycin Conjugate Efficiently Eradicates Intracellular Bacteria via a Dual Mechanism

Infections by intracellular pathogens are difficult to treat because of the poor accessibility of antibiotics to the pathogens encased by host cell membranes. As such, a strategy that can improve the membrane permeability of antibiotics would significantly increase their efficiency against the intracellular pathogens. Here, we report the design of an adaptive, metaphilic cell-penetrating polypeptide (CPP)-antibiotic conjugate (VPP-G) that can effectively eradicate the intracellular bacteria both in vitro and in vivo. VPP-G was synthesized by attaching vancomycin to a highly membrane-penetrative guanidinium-functionalized metaphilic CPP. VPP-G effectively kills not only extracellular but also far more challenging intracellular pathogens, such as S. aureus, methicillin-resistant S. aureus, and vancomycin-resistant Enterococci. VPP-G enters the host cell via a unique metaphilic membrane penetration mechanism and kills intracellular bacteria through disruption of both cell wall biosynthesis and membrane integrity. This dual antimicrobial mechanism of VPP-G prevents bacteria from developing drug resistance and could also potentially kill dormant intracellular bacteria. VPP-G effectively eradicates MRSA in vivo, significantly outperforming vancomycin, which represents one of the most effective intracellular antibacterial agents reported so far. This strategy can be easily adapted to develop other conjugates against different intracellular pathogens by attaching different antibiotics to these highly membrane-penetrative metaphilic CPPs.

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.

Targeted and Intracellular Antibacterial Activity against S. agalactiae of the Chimeric Peptides Based on Pheromone and Cell-Penetrating Peptides

The significance of the complex bacterial ecosystem in the human body and the impediment of the mammalian membrane against many antibiotics together emphasize the necessity to develop antimicrobial agents with precise antimicrobial and cell-penetrating activities. A simple and feasible method for generating dual-function antimicrobial peptides inspired by highly hydrophobic peptide pheromone and cationic cell-penetrating peptides is presented. Furthermore, the extension of the peptide candidate library is achieved by modifying the charged domain. The bacteria-selective peptides L1, L2, L10, and L11 kill Streptococcus agalactiae by disrupting the membrane structure, and the targeted mechanism is suggested where the peptides offset the entrapment of S. agalactiae rather than of other bacteria. Moreover, L2 and L10 possess intracellular antibacterial activity and carrier property, which is mainly dependent on endocytosis. Given their suitable biocompatibility, high tolerance, no drug resistance, and effective antimicrobial capacity in a mouse mastitis model, L2 and L10 can be powerful weapons against S. agalactiae pathogen infection.

Enhancing Aqueous Solubility and Antibacterial Activity of Curcumin by Complexing with Cell-Penetrating Octaarginine

Bacterial resistance to antimicrobial drugs is one of the biggest threats to human health and novel drugs, and strategies are needed to obviate this resistance crisis. An innovative strategy for designing novel antimicrobial drugs is based on the hybridization of an antimicrobial agent with a second functional entity. Here, we use a cell-penetrating peptide-octaarginine (R8) as the second functional entity and develop a complex or hybrid of R8 and curcumin that possibly targets the bacterial cell membrane. Minimum inhibitory concentration assays show that the antibacterial activity of the complex is enhanced in a synergistic manner and rapid killing kinetics are obtained, emphasizing a bactericidal mode of action. In addition, electron microscopy images reveal bacterial membrane disruption by the complex. The R8-curcumin complex also displays activity against HeLa cells.

Cell Penetrating Peptides, Novel Vectors for Gene Therapy

Cell penetrating peptides (CPPs), also known as protein transduction domains (PTDs), first identified ~25 years ago, are small, 6-30 amino acid long, synthetic, or naturally occurring peptides, able to carry variety of cargoes across the cellular membranes in an intact, functional form. Since their initial description and characterization, the field of cell penetrating peptides as vectors has exploded. The cargoes they can deliver range from other small peptides, full-length proteins, nucleic acids including RNA and DNA, liposomes, nanoparticles, and viral particles as well as radioisotopes and other fluorescent probes for imaging purposes. In this review, we will focus briefly on their history, classification system, and mechanism of transduction followed by a summary of the existing literature on use of CPPs as gene delivery vectors either in the form of modified viruses, plasmid DNA, small interfering RNA, oligonucleotides, full-length genes, DNA origami or peptide nucleic acids.

Abiotic Factors Promote Cell Penetrating Peptide Permeability in Enterobacteriaceae Models

Conventionally, the delivery of biomolecules into bacteria for the generation of characterized or functional mutants has relied greatly on horizontal gene transfer techniques. However, the low compatibility of these techniques with novel or hard-to-transform bacteria currently serves as a challenge to the bioengineering field. Here, we explored the use of cell penetrating peptides (CPPs) as an alternative biomolecule delivery approach by investigating the effects of the abiotic factors during CPP permeation. Using the (KFF)₃K-FAM conjugate and Escherichia coli as models, we evaluated four abiotic factors where two of these factors, temperature and solution tonicity, promoted (KFF)₃K-FAM permeation efficiency. Our data show that optimal (KFF)₃K-FAM permeation efficiency was achieved for E. coli at approximately 98.1% under conditions of 37°C (growth optimal temperature) and 50% PBS concentration. Based on these conditions, we subsequently tested the applicability of CPP permeation in various bacterial strains by treating 10 bacterial strains from the Enterobacteriaceae family among which seven strains have no CPP permeation records with (KFF)₃K-FAM. Interestingly, when compared with non-optimized conditions, all 10 strains showed a marked increase in CPP permeation ranging between 20 and 90% efficiency. Although using strains within Enterobacteriaceae that are phylogenetically close, our results hinted on the possibility that with proper optimization of the abiotic factors, CPPs could be compatible with a broad range of bacterial strains. Our efforts suggest that CPP could serve as an effective alternative approach for mutant generation and for biomolecule delivery into novel or hard-to-transform bacteria.

Inhibitors of Intracellular Gram-Positive Bacterial Growth Synthesized via Povarov-Doebner Reactions

Staphylococcus aureus can survive both inside and outside of phagocytic and nonphagocytic host cells. Once in the intracellular milieu, most antibiotics have reduced ability to kill S. aureus, thus resulting in relapse of infection. Consequently, there is a need for antibacterial agents that can accumulate to lethal concentrations within host cells to clear intracellular infections. We have identified tetrahydrobenzo[a or c]phenanthridine and tetrahydrobenzo[a or c]acridine compounds, synthesized via a one-flask Povarov-Doebner operation from readily available amines, aldehydes, and cyclic ketones, as potent agents against drug-resistant S. aureus. Importantly, the tetrahydrobenzo[a or c]phenanthridine and tetrahydrobenzo[a or c]acridine compounds can accumulate in macrophage cells and reduce the burden of intracellular MRSA better than the drug of choice, vancomycin. We observed that MRSA could not develop resistance (by passage 30) against tetrahydrobenzo[a or c]acridine compound 15. Moreover, tetrahydrobenzo[c]acridine compound 15 and tetrahydrobenzo[c]phenanthridine compound 16 were nontoxic to red blood cells and were nonmutagenic. Preliminary data indicated that compound 16 reduced bacterial load (MRSA USA300) in mice (thigh infection model) to the same degree as vancomycin. These observations suggest that compounds 15 and 16 and analogues thereof could become therapeutic agents for the treatment of chronic MRSA infections.

How to evaluate the cellular uptake of CPPs with fluorescence techniques: Dissecting methodological pitfalls associated to tryptophan-rich peptides

Cell-penetrating peptides (CPP) are broadly recognized as efficient non-viral vectors for the internalization of compounds such as peptides, oligonucleotides or proteins. Characterizing these carriers requires reliable methods to quantify their intracellular uptake. Flow cytometry on living cells is a method of choice but is not always applicable (e.g. big or polarized cells), so we decided to compare it to fluorescence spectroscopy on cell lysates. Surprisingly, for the internalization of a series of TAMRA-labeled conjugates formed of either cationic or amphipathic CPPs covalently coupled to a decamer peptide, we observed important differences in internalization levels between both methods. We partly explained these discrepancies by analyzing the effect of buffer conditions (pH, detergents) and peptide sequence/structure on TAMRA dye accessibility. Based on this analysis, we calculated a correction coefficient allowing a better coherence between both methods. However, an overestimated signal was still observable for both amphipathic peptides using the spectroscopic detection, which could be due to their localization at the cell membrane. Based on several in vitro experiments modeling events at the plasma membrane, we hypothesized that fluorescence of peptides entrapped in the membrane bilayer could be quenched by the tryptophan residues of close transmembrane proteins. During cell lysis, cell membranes are disintegrated liberating the entrapped peptides and restoring the fluorescence, explaining the divergences observed between flow cytometry and spectroscopy on lysates. Overall, our results highlighted major biases in the fluorescently-based quantification of internalized fluorescently-labeled CPP conjugates, which should be considered for accurate uptake quantification.

Internalization, distribution, and activity of peptide H2 against the intracellular multidrug-resistant bovine mastitis-causing bacterium Staphylococcus aureus

Bovine mastitis is mainly caused by Staphylococcus aureus, which is difficult to eliminate, prone to escape from antibacterial agents, and may cause recurring infections due to the intracellular nature of its infection and multidrug resistance. In this study, the intracellular activities of the NZ2114 derivative peptide H18R (H2) against methicillin-resistant S. aureus (MRSA) and multidrug-resistant bovine S. aureus strains were investigated in bovine mammary epithelial MAC-T cells and mouse mammary glands. The minimum inhibitory concentrations of H2 against S. aureus were 0.5‒1 μg/ml; H2 displayed a lower cytotoxicity than its parental peptide NZ2114 (survival rates of MAC-T cells: 100% [H2 treatment] vs 60.7% [NZ2114 (256 μg/ml) treatment]). H2 was internalized into MAC-T cells mainly via clathrin-mediated endocytosis, and distributed in the cytoplasm. The intracellular inhibition rates against MRSA ATCC43300, the mastitis isolates S. aureus CVCC 3051 and E48 were above 99%, 99%, and 94%, respectively; these were higher than those in case of vancomycin (23-47%). In the mouse model of S. aureus E48-induced mastitis, after treatment with 100 μg of H2 and vancomycin, bacterial numbers in each mammary gland were reduced by 3.96- and 1.59-log CFU, respectively. Additionally, similar to NZ2114 and vancomycin, H2 alleviated the histopathological damage of the mammary tissue and polymorphonuclear neutrophil infiltration in the alveoli. These results suggest that H2 can be used as a safe and effective candidate for treating S. aureus-induced mastitis.

Drug delivery systems designed to overcome antimicrobial resistance

Antimicrobial resistance has emerged as a huge challenge to the effective treatment of infectious diseases. Aside from a modest number of novel anti-infective agents, very few new classes of antibiotics have been successfully developed for therapeutic use. Despite the research efforts of numerous scientists, the fight against antimicrobial (ATB) resistance has been a longstanding continued effort, as pathogens rapidly adapt and evolve through various strategies, to escape the action of ATBs. Among other mechanisms of resistance to antibiotics, the sophisticated envelopes surrounding microbes especially form a major barrier for almost all anti-infective agents. In addition, the mammalian cell membrane presents another obstacle to the ATBs that target intracellular pathogens. To negotiate these biological membranes, scientists have developed drug delivery systems to help drugs traverse the cell wall; these are called "Trojan horse" strategies. Within these delivery systems, ATB molecules can be conjugated with one of many different types of carriers. These carriers could include any of the following: siderophores, antimicrobial peptides, cell-penetrating peptides, antibodies, or even nanoparticles. In recent years, the Trojan horse-inspired delivery systems have been increasingly reported as efficient strategies to expand the arsenal of therapeutic solutions and/or reinforce the effectiveness of conventional ATBs against drug-resistant microbes, while also minimizing the side effects of these drugs. In this paper, we aim to review and report on the recent progress made in these newly prevalent ATB delivery strategies, within the current context of increasing ATB resistance.

Trapped! A Critical Evaluation of Methods for Measuring Total Cellular Uptake versus Cytosolic Localization

Biomolecules have many properties that make them promising for intracellular therapeutic applications, but delivery remains a key challenge because large biomolecules cannot easily enter the cytosol. Furthermore, quantification of total intracellular versus cytosolic concentrations remains demanding, and the determination of delivery efficiency is thus not straightforward. In this review, we discuss strategies for delivering biomolecules into the cytosol and briefly summarize the mechanisms of uptake for these systems. We then describe commonly used methods to measure total cellular uptake and, more selectively, cytosolic localization, and discuss the major advantages and drawbacks of each method. We critically evaluate methods of measuring "cell penetration" that do not adequately distinguish total cellular uptake and cytosolic localization, which often lead to inaccurate interpretations of a molecule's cytosolic localization. Finally, we summarize the properties and components of each method, including the main caveats of each, to allow for informed decisions about method selection for specific applications. When applied correctly and interpreted carefully, methods for quantifying cytosolic localization offer valuable insight into the bioactivity of biomolecules and potentially the prospects for their eventual development into therapeutics.

Transportan 10 improves the pharmacokinetics and pharmacodynamics of vancomycin

In the presented study, transportan 10 (TP10), an amphipathic cell penetrating peptide (CPP) with high translocation activity, was conjugated with vancomycin (Van), which is known for poor access to the intracellular bacteria and the brain. The antibacterial activity of the conjugates was tested on selected clinical strains of methicillin-resistant Staphylococcus aureus (MRSA) and Enterococcus sp. It turned out that all of them had superior antimicrobial activity in comparison to that of free Van, which became visible particularly against clinical MRSA strains. Furthermore, one of the conjugates was tested against MRSA - infected human cells. With respect to them, this compound showed high bactericidal activity. Next, the same conjugate was screened for its capacity to cross the blood brain barrier (BBB). Therefore, qualitative and quantitative analyses of the conjugate's presence in the mouse brain slices were carried out after its iv administration. They indicated the conjugate's presence in the brain in amount >200 times bigger than that of Van. The conjugates were safe with respect to erythrocyte toxicity (erythrocyte lysis assay). Van in the form of a conjugate with TP10 acquires superior pharmacodynamic and pharmacokinetic.

The dual functionality of antimicrobial peptides Os and Os-C in human leukocytes

Antimicrobial peptides (AMPs), Os and Os-C, have been identified as multifunctional peptides with antibacterial, antiendotoxin, and anti-inflammatory properties. For further development of Os and Os-C as therapeutic peptides, it is essential to evaluate these effects in human mononuclear (MN) and polymorphonuclear (PMN) leukocytes. The cytotoxicity and the effects of both peptides on MN and PMN morphology were determined with the Alamar-Blue assay and scanning electron microscopy, respectively. The ability of Os and Os-C to induce reactive oxygen species (ROS) and to protect against 2,2'-azobis(2-amidinopropane) dihydrochloride-induced oxidative damage in both cell populations was evaluated using 2',7'-dichlorofluorescin diacetate (DCFH-DA). Using fluorescently labeled peptides, the ability of the peptides to cross the cell membranes of MN and PMN was also evaluated. At the minimum bactericidal concentrations of Os and Os-C, neither peptide was cytotoxic. Os caused morphological features of toxicity at 100 μM, entered MN cells, and also protected these cells against oxidative damage. Os-C caused MN and PMN leukocyte activation associated with ROS formation and was unable to penetrate cell membranes, indicating extracellular membrane interactions. This study confirms that both Os and Os-C at less than 100 μM are not cytotoxic. The MN-specific uptake of Os identifies it as a cell-specific cargo-carrier peptide, with additional anti-inflammatory properties. In contrast, the ability of Os-C to activate MN and PMN cells implies that this peptide should be further evaluated as an AMP, which, in addition to its ability to eradicate infection, can further enhance host immunity. These novel characteristics of Os and Os-C indicate that these AMPs as peptides can be further developed for specific applications.

Improved Antibacterial Activity of the Marine Peptide N6 against Intracellular Salmonella Typhimurium by Conjugating with the Cell-Penetrating Peptide Tat11 via a Cleavable Linker

The poor penetration ability of antimicrobial agents limits their use in the treatment of intracellular bacteria. In this study, the conjugate CNC (6) was generated by connecting the cell-penetrating peptide Tat₁₁ (1) and marine peptide N6 (2) via a cathepsin-cleavable linker, and the C-terminal aminated N6 (7) and CNC (8) were first designed and synthesized to eliminate intracellular Salmonellae Typhimurium. The cellular uptake of 6 and stability of 7 were higher than those of 2, and conjugates 6, 8, and 7 had almost no hemolysis and cytotoxicity. The antibacterial activities of 6, 8, and 7 against S. Typhimurium in RAW264.7 cells were increased by 67.2-76.2%, 98.6-98.9%, and 96.3-97.6%, respectively. After treatment with 1-2 μmol/kg of 6, 8, or 7, the survival of the S. Typhimurium-infected mice was 66.7-100%, higher than that of 2 (33.4-66.7%). This result suggested that 6, 8, and 7 may be excellent candidates for novel antimicrobial agents to treat intracellular pathogens.

Bacterial LPX motif-harboring virulence factors constitute a species-spanning family of cell-penetrating effectors

Effector proteins are key virulence factors of pathogenic bacteria that target and subvert the functions of essential host defense mechanisms. Typically, these proteins are delivered into infected host cells via the type III secretion system (T3SS). Recently, however, several effector proteins have been found to enter host cells in a T3SS-independent manner thereby widening the potential range of these virulence factors. Prototypes of such bacteria-derived cell-penetrating effectors (CPEs) are the Yersinia enterocolitica-derived YopM as well as the Salmonella typhimurium effector SspH1. Here, we investigated specifically the group of bacterial LPX effector proteins comprising the Shigella IpaH proteins, which constitute a subtype of the leucine-rich repeat protein family and share significant homologies in sequence and structure. With particular emphasis on the Shigella-effector IpaH9.8, uptake into eukaryotic cell lines was shown. Recombinant IpaH9.8 (rIpaH9.8) is internalized via endocytic mechanisms and follows the endo-lysosomal pathway before escaping into the cytosol. The N-terminal alpha-helical domain of IpaH9.8 was identified as the protein transduction domain required for its CPE ability as well as for being able to deliver other proteinaceous cargo. rIpaH9.8 is functional as an ubiquitin E3 ligase and targets NEMO for poly-ubiquitination upon cell penetration. Strikingly, we could also detect other recombinant LPX effector proteins from Shigella and Salmonella intracellularly when applied to eukaryotic cells. In this study, we provide further evidence for the general concept of T3SS-independent translocation by identifying novel cell-penetrating features of these LPX effectors revealing an abundant species-spanning family of CPE.

Prediction of Cell-Penetrating Potential of Modified Peptides Containing Natural and Chemically Modified Residues

Designing drug delivery vehicles using cell-penetrating peptides is a hot area of research in the field of medicine. In the past, number of in silico methods have been developed for predicting cell-penetrating property of peptides containing natural residues. In this study, first time attempt has been made to predict cell-penetrating property of peptides containing natural and modified residues. The dataset used to develop prediction models, include structure and sequence of 732 chemically modified cell-penetrating peptides and an equal number of non-cell penetrating peptides. We analyzed the structure of both class of peptides and observed that positive charge groups, atoms, and residues are preferred in cell-penetrating peptides. In this study, models were developed to predict cell-penetrating peptides from its tertiary structure using a wide range of descriptors (2D, 3D descriptors, and fingerprints). Random Forest model developed by using PaDEL descriptors (combination of 2D, 3D, and fingerprints) achieved maximum accuracy of 95.10%, MCC of 0.90 and AUROC of 0.99 on the main dataset. The performance of model was also evaluated on validation/independent dataset which achieved AUROC of 0.98. In order to assist the scientific community, we have developed a web server “CellPPDMod” for predicting the cell-penetrating property of modified peptides (http://webs.iiitd.edu.in/raghava/cellppdmod/).

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.

Increased intracellular activity of MP1102 and NZ2114 against Staphylococcus aureus in vitro and in vivo

Treatment of Staphylococcus aureus infections remains very difficult due to its capacity to survive intracellularly and its multidrug resistance. In this study, the extracellular/intracellular activities of plectasin derivatives-MP1102/NZ2114 were investigated against three methicillin-susceptible/-resistant S. aureus (MSSA/MRSA) strains in RAW 264.7 macrophages and mice to overcome poor intracellular activity. Antibacterial activities decreased 4–16-fold under a mimic phagolysosomal environment. MP1102/NZ2114 were internalized into the cells via clathrin-mediated endocytosis and macropinocytosis and distributed in the cytoplasm; they regulated tumor necrosis factor-α, interleukin-1β and interleukin-10 levels. The extracellular maximal relative efficacy (E_max) values of MP1102/NZ2114 towards the three S. aureus strains were >5-log decrease in colony forming units (CFU). In the methicillin-resistant and virulent strains, MP1102/NZ2114 exhibited intracellular bacteriostatic efficacy with an E_max of 0.42–1.07-log CFU reduction. In the MSSA ATCC25923 mouse peritonitis model, 5 mg/kg MP1102/NZ2114 significantly reduced the bacterial load at 24 h, which was superior to vancomycin. In MRSA ATCC43300, their activity was similar to that of vancomycin. The high virulent CVCC546 strain displayed a relatively lower efficiency, with log CFU decreases of 2.88–2.91 (total), 3.41–3.50 (extracellular) and 2.11–2.51 (intracellular) compared with vancomycin (3.70). This suggests that MP1102/NZ2114 can be used as candidates for treating intracellular S. aureus.

Targeted transport of nanocarriers into brain for theranosis with rabies virus glycoprotein-derived peptide

For successful theranosis of brain diseases, limited access of therapeutic molecules across blood-brain barrier (BBB) needs be overcome in brain delivery. Currently, peptide derivatives of rabies virus glycoprotein (RVG) have been exploited as delivery ligands to transport nanocarriers across BBB and specifically into the brain. The targeting peptides usually conjugate to the nanocarrier surface, and the cargoes, including siRNA, miRNA, DNA, proteins and small molecular chemicals, are complexed or encapsulated in the nanocarriers. The peptide ligand of the RVG-modified nanocarriers introduces the conjugated targeted-delivery into the brain, and the cargoes are involved in disease theranosis. The peptide-modified nanocarriers have been applied to diagnose and treat various brain diseases, such as glioma, Alzheimer's disease, ischemic injury, protein misfolding diseases etc. Since the targeting delivery system has displayed good biocompatibility and desirable therapeutic effect, it will raise a potential application in treating brain diseases.

Improved antibacterial activity of a marine peptide-N2 against intracellular Salmonella typhimurium by conjugating with cell-penetrating peptides-bLFcin6/Tat11

Salmonellae, gram-negative bacteria, are facultative intracellular pathogens that cause a number of diseases in animals and humans. The poor penetration ability of antimicrobial agents limits their use in the treatment of intracellular bacterial infections. In this study, the cell-penetrating peptides (CPPs) bLFcin₆ and Tat₁₁ were separately conjugated to the antimicrobial peptide N2, and the antibacterial activity and pharmacodynamics of the CPPs-N2 conjugates were first evaluated against Salmonellae typhimurium in vitro and in macrophage cells. The cytotoxicity, cellular uptake and mechanism of cellular internalization of the CPPs-N2 conjugates were also examined in RAW264.7 cells. Similar to N2, CPPs-N2 have two reverse β-sheets and three loops. The minimal inhibitory concentration (MIC) of CPPs-N2 was approximately 2 μM, which was higher than that of N2 (0.8 μM). The dose-time curves and cytotoxicity assay showed that both peptide conjugates were more effective than N2 alone at concentrations ranging from 0.25 to 1 × MIC, and they exhibited low cytotoxicity (9.78%-13.54%) at 100 μM. After 0.5 h incubation, the cell internalization ratio of B6N2 and T11N2 exceeded 28.3% and 93.5%, respectively, which was higher than that of N2. The uptake of B6N2 and T11N2 was reduced by low temperature (82.1%-91.7%), chlorpromazine (35.7%-75.1%), and amiloride (26.0%-52.1%), indicating that macropinocytosis and clathrin-mediated endocytosis may be involved. Approximately 98.85% and 91.35% of bacteria were killed within 3 h by T11N2 and B6N2, respectively, which was higher than the percentage killed by N2 (69.74%). Compared with the bactericidal activity of N2 alone, the bactericidal activity of T11N2 and B6N2 was increased by 53.7%-99.6% and 85.3-85.8%, respectively. Both CPPs-N2 conjugates may be excellent candidates for novel antimicrobial agents to treat infectious diseases caused by intracellular pathogens.

Cell-penetrating peptides derived from Clostridium difficile TcdB2 and a related large clostridial toxin

Clostridium difficile TcdB (2366 amino acid residues) is an intracellular bacterial toxin that binds to cells and enters the cytosol where it glucosylates small GTPases. In the current study, we examined a putative cell entry region of TcdB (amino acid residues 1753-1851) for short sequences that function as cell-penetrating peptides (CPPs). To screen for TcdB-derived CPPs, a panel of synthetic peptides was tested for the ability to enhance transferrin (Tf) association with cells. Four candidate CPPs were discovered, and further study on one peptide (PepB2) pinpointed an asparagine residue necessary for CPP activity. PepB2 mediated the cell entry of a wide variety of molecules including dextran, streptavidin, microspheres, and lentivirus particles. Of note, this uptake was dramatically reduced in the presence of the Na⁺/H⁺ exchange blocker and micropinocytosis inhibitor amiloride, suggesting that PepB2 invokes macropinocytosis. Moreover, we found that PepB2 had more efficient cell-penetrating activity than several other well-known CPPs (TAT, penetratin, Pep-1, and TP10). Finally, Tf assay-based screening of peptides derived from two other large clostridial toxins, TcdA and TcsL, uncovered two new TcdA-derived CPPs. In conclusion, we have identified six CPPs from large clostridial toxins and have demonstrated the ability of PepB2 to promote cell association and entry of several molecules through a putative fluid-phase macropinocytotic mechanism.