Nucleic acid delivery by cell penetrating peptides derived from dengue virus capsid protein: design and mechanism of action

Design of Natterins-based peptides improves antimicrobial and antiviral activities

The biochemical analysis of animal venoms has been intensifying over the years, enabling the prediction of new molecules derived from toxins, harnessing the therapeutic potential of these molecules. From the venom of the fish Thalassophryne nattereri, using in silico methods for predicting antimicrobial and cell-penetrating peptides, two peptides from Natterins with promising characteristics were synthesized and subjected to in vitro and in vivo analysis. The peptides were subjected to stability tests and antimicrobial assays, cytotoxicity in murine fibroblast cells, antiviral assays against the Chikungunya virus, and the toxicity on G. mellonella was also evaluated. The findings underscore the peptides' robust stability under varying temperatures and pH conditions and resistance to proteolytic degradation. The peptides demonstrated significant antimicrobial efficacy, minimal cytotoxicity, and low hemolytic activity. Although their antiviral efficacy was limited, they showed potential at specific stages of viral replication. The in vivo toxicity tests indicated a favorable safety profile. These findings suggest that this approach can aid in the development of antimicrobial agents, offering a faster and personalized method to combat microbial infections, and represent a promising discovery in venom biotechnology research.

A comprehensive overview on the crosstalk between microRNAs and viral pathogenesis and infection

Infections caused by viruses as the smallest infectious agents, pose a major threat to global public health. Viral infections utilize different host mechanisms to facilitate their own propagation and pathogenesis. MicroRNAs (miRNAs), as small noncoding RNA molecules, play important regulatory roles in different diseases, including viral infections. They can promote or inhibit viral infection and have a pro-viral or antiviral role. Also, viral infections can modulate the expression of host miRNAs. Furthermore, viruses from different families evade the host immune response by producing their own miRNAs called viral miRNAs (v-miRNAs). Understanding the replication cycle of viruses and their relation with host miRNAs and v-miRNAs can help to find new treatments against viral infections. In this review, we aim to outline the structure, genome, and replication cycle of various viruses including hepatitis B, hepatitis C, influenza A virus, coronavirus, human immunodeficiency virus, human papillomavirus, herpes simplex virus, Epstein-Barr virus, Dengue virus, Zika virus, and Ebola virus. We also discuss the role of different host miRNAs and v-miRNAs and their role in the pathogenesis of these viral infections.

Exploring the Chemical Features and Biomedical Relevance of Cell-Penetrating Peptides

Cell-penetrating peptides (CPPs) are a diverse group of peptides, typically composed of 4 to 40 amino acids, known for their unique ability to transport a wide range of substances-such as small molecules, plasmid DNA, small interfering RNA, proteins, viruses, and nanoparticles-across cellular membranes while preserving the integrity of the cargo. CPPs exhibit passive and non-selective behavior, often requiring functionalization or chemical modification to enhance their specificity and efficacy. The precise mechanisms governing the cellular uptake of CPPs remain ambiguous; however, electrostatic interactions between positively charged amino acids and negatively charged glycosaminoglycans on the membrane, particularly heparan sulfate proteoglycans, are considered the initial crucial step for CPP uptake. Clinical trials have highlighted the potential of CPPs in diagnosing and treating various diseases, including cancer, central nervous system disorders, eye disorders, and diabetes. This review provides a comprehensive overview of CPP classifications, potential applications, transduction mechanisms, and the most relevant algorithms to improve the accuracy and reliability of predictions in CPP development.

MeCP2 is a naturally supercharged protein with cell membrane transduction capabilities

The intrinsically disordered protein MeCP2 is a global transcriptional regulator encoded by the MECP2 gene. Although the structured domains of MeCP2 have been the subject of multiple studies, its unstructured regions have not been that extensively characterized. In this work, we show that MeCP2 possesses properties akin to those of supercharged proteins. By utilizing its unstructured portions, MeCP2 can successfully transduce across cell membranes and localize to heterochromatic foci in the nuclei, displaying uptake levels a third lower than a MeCP2 construct fused to the cell-penetrating peptide TAT. MeCP2 uptake can further be enhanced by the addition of compounds that promote endosomal escape following cellular trafficking by means of macropinocytosis. Using a combination of in silico prediction algorithms and live-cell imaging experiments, we mapped the sequence in MeCP2 responsible for its cellular incorporation, which bears a striking resemblance to TAT itself. Transduced MeCP2 was shown to interact with HDAC3. These findings provide valuable insight into the properties of MeCP2 and may be beneficial for devising future protein-based treatment strategies.

Cell Penetrating Peptides: Classification, Mechanisms, Methods of Study, and Applications

Cell-penetrating peptides (CPPs) encompass a class of peptides that possess the remarkable ability to cross cell membranes and deliver various types of cargoes, including drugs, nucleic acids, and proteins, into cells. For this reason, CPPs are largely investigated in drug delivery applications in the context of many diseases, such as cancer, diabetes, and genetic disorders. While sharing this functionality and some common structural features, such as a high content of positively charged amino acids, CPPs represent an extremely diverse group of elements, which can differentiate under many aspects. In this review, we summarize the most common characteristics of CPPs, introduce their main distinctive features, mechanistic aspects that drive their function, and outline the most widely used techniques for their structural and functional studies. We highlight current gaps and future perspectives in this field, which have the potential to significantly impact the future field of drug delivery and therapeutics.

Cell-Penetrating Peptide-Based Delivery of Macromolecular Drugs: Development, Strategies, and Progress

Cell-penetrating peptides (CPPs), developed for more than 30 years, are still being extensively studied due to their excellent delivery performance. Compared with other delivery vehicles, CPPs hold promise for delivering different types of drugs. Here, we review the development process of CPPs and summarize the composition and classification of the CPP-based delivery systems, cellular uptake mechanisms, influencing factors, and biological barriers. We also summarize the optimization routes of CPP-based macromolecular drug delivery from stability and targeting perspectives. Strategies for enhanced endosomal escape, which prolong its half-life in blood, improved targeting efficiency and stimuli-responsive design are comprehensively summarized for CPP-based macromolecule delivery. Finally, after concluding the clinical trials of CPP-based drug delivery systems, we extracted the necessary conditions for a successful CPP-based delivery system. This review provides the latest framework for the CPP-based delivery of macromolecular drugs and summarizes the optimized strategies to improve delivery efficiency.

The antimetastatic breast cancer activity of the viral protein-derived peptide vCPP2319 as revealed by cellular biomechanics

The incidence of metastatic breast cancer (MBC) is increasing and the therapeutic arsenal available to fight it is insufficient. Brain metastases, in particular, represent a major challenge for chemotherapy as the impermeable nature of the blood-brain barrier (BBB) prevents most drugs from targeting cells in the brain. For their ability to transpose biological membranes and transport a broad spectrum of bioactive cargoes, cell-penetrating peptides (CPPs) have been hailed as ideal candidates to deliver drugs across biological barriers. A more ambitious approach is to have the CPP as a drug itself, capable of both killing cancer cells and interacting with the blood/brain interface, therefore blocking the onset of brain metastases. vCPP2319, a viral protein-derived CPP, has both properties as it: (a) is selective toward human breast cancer cells (MDA-MB-231) and increases cell stiffness compared to breast epithelial cells (MCF 10A) hindering the progression of metastases; and (b) adsorbs at the surface of human brain endothelial cells potentially counteracting metastatic cells from reaching the brain. Overall, the results reveal the selective anticancer activity of the peptide vCPP2319, which is also able to reside at the blood-brain interface, therefore counteracting brain penetration by metastatic cancer cells.

Emerging landscape of cell-penetrating peptide-mediated nucleic acid delivery and their utility in imaging, gene-editing, and RNA-sequencing

The safety issues like immunogenicity and unacceptable cancer risk of viral vectors for DNA/mRNA vaccine delivery necessitate the development of non-viral vectors with no toxicity. Among the non-viral strategies, cell-penetrating peptides (CPPs) have been a topic of interest recently because of their ability to cross plasma membranes and facilitate nucleic acids delivery both in vivo and in vitro. In addition to the application in the field of gene vaccine and gene therapy, CPPs based nucleic acids delivery have been proved by its potential application like gene editing, RNA-sequencing, and imaging. Here, we focus on summarizing the recent applications and progress of CPPs-mediated nucleic acids delivery and discuss the current problems and solutions in this field.

Dual-Functional Peptide Driven Liposome Codelivery System for Efficient Treatment of Doxorubicin-Resistant Breast Cancer

BACKGROUND

The active-targeted drug delivery systems had attracted more and more attention to efficiently overcome multidrug resistance (MDR) in cancer treatments. The aim of the work was to develop a multifunctional nano-structured liposomal system for co-delivery of doxorubicin hydrochloride (DOX) and celecoxib (CEL) to overcome doxorubicin resistance in breast cancer.

METHODS

A functional hybrid peptide (MTS-R8H3) with unique cellular penetrability, endo-lysosomal escape and mitochondrial targeting ability was successfully synthesized using solid phase synthesis technology. The peptide modified targeted liposomes (DOX/CEL-MTS-R8H3 lipo) for co-delivery of DOX and CEL were formulated to overcome the chemoresistance in MCF/ADR cells.

RESULTS

DOX/CEL-MTS-R8H3 lipo showed nanosized shape and displayed high stability for one month. The cytotoxicity effect of the co-delivery of DOX and CEL through peptide modified liposomes had remarkable treatment efficacy on killing MCF/ADR cells. Targeted liposome exhibited greater cellular entry ability about 5.72-fold stronger than DOX solution. Moreover, as compared with unmodified liposomes, the presence of MTS-R8H3 peptide entity on liposome surface enhanced the mitochondrial-targeting ability and achieved effective reactive oxygen species (ROS) production with significant inhibition of P-gp efflux activity.

CONCLUSION

The study suggested that the DOX/CEL-MTS-R8H3 lipo is a promising strategy for overcoming drug resistance in breast cancer treatments with high targeting inhibition efficiency.

Cell-penetrating peptides in the intracellular delivery of viral nanoparticles

Cell-penetrating peptides (CPPs) are a promising tool for the intracellular delivery of cargo. Due to their ability to cross membranes while also cotransporting various cargoes, they offer great potential for biomedical applications. Several CPPs have been derived from viral proteins with natural roles in the viral replication cycle that require them to breach or fuse to cellular membranes. Additionally, the ability of viruses to cross membranes makes viruses and virus-based particles a convenient model for research on nanoparticle delivery and nanoparticle-mediated gene therapy. In this chapter, we aim to characterize CPPs derived from both structural and nonstructural viral proteins. Their function as enhancers of viral infection and transduction by viral nanoparticles as well as the main features of viral CPPs employed in intracellular cargo delivery are summarized to emphasize their potential use in nanomedicine.

Structure and function of capsid protein in flavivirus infection and its applications in the development of vaccines and therapeutics

Flaviviruses are enveloped single positive-stranded RNA viruses. The capsid (C), a structural protein of flavivirus, is dimeric and alpha-helical, with several special structural and functional features. The functions of the C protein go far beyond a structural role in virions. It is not only responsible for encapsidation to protect the viral RNA but also able to interact with various host proteins to promote virus proliferation. Therefore, the C protein plays an important role in infected host cells and the viral life cycle. Flaviviruses have been shown to affect the health of humans and animals. Thus, there is an urgent need to effectively control flavivirus infections. The structure of the flavivirus virion has been determined, but there is relatively little information about the function of the C protein. Hence, a greater understanding of the role of the C protein in viral infections will help to discover novel antiviral strategies and provide a promising starting point for the further development of flavivirus vaccines or therapeutics.

Role of nuclear localization signals in the DNA delivery function of Chikungunya virus capsid protein

Capsids of several RNA viruses are reported to have unconventional roles attributed to their subcellular trafficking property. The capsid of CHIKV is also found to localize in the nucleus, but the rationale is not yet clear. To understand the role of the nuclear-localized capsid, we examined the nucleic acid binding and cargo delivery activity of the CHIKV capsid. We used bacterially purified capsid protein to probe the binding affinity with CHIKV genome-specific and non-specific nucleic acids. We found that the capsid was able to bind non-specifically to different forms of nucleic acids. The successful transfection of GFP-tagged plasmid DNA by CHIKV capsid protein shows the DNA delivery ability of the protein. Further, we selected and investigated the DNA binding and cargo delivery activity of commercially synthesized Nuclear Localization Signal sequences (NLS 1 and NLS2) of capsid protein. Both peptides showed comparable DNA binding affinity, however, only the NLS1 peptide was capable of delivering plasmid DNA inside the cell. Furthermore, the cellular uptake study using the FITC-labelled NLS1 peptide was performed to highlight the membrane penetrating ability. Structural analysis was performed using circular dichroism and NMR spectroscopy to elucidate the transfection ability of the NLS1 peptides. Our findings suggest that the capsid of CHIKV might influence cellular trafficking in the infected cell via non-specific interactions. Our study also indicates the significance of NLS sequences in the multifunctionality of CHIKV capsid protein.

Exploring novel and potent cell penetrating peptides in the proteome of SARS-COV-2 using bioinformatics approaches

Among various delivery systems for vaccine and drug delivery, cell-penetrating peptides (CPPs) have been known as a potent delivery system because of their capability to penetrate cell membranes and deliver some types of cargoes into cells. Several CPPs were found in the proteome of viruses such as Tat originated from human immunodeficiency virus-1 (HIV-1), and VP22 derived from herpes simplex virus-1 (HSV-1). In the current study, a wide-range of CPPs was identified in the proteome of SARS-CoV-2, a new member of coronaviruses family, using in silico analyses. These CPPs may play a main role for high penetration of virus into cells and infection of host. At first, we submitted the proteome of SARS-CoV-2 to CellPPD web server that resulted in a huge number of CPPs with ten residues in length. Afterward, we submitted the predicted CPPs to C2Pred web server for evaluation of the probability of each peptide. Then, the uptake efficiency of each peptide was investigated using CPPred-RF and MLCPP web servers. Next, the physicochemical properties of the predicted CPPs including net charge, theoretical isoelectric point (pI), amphipathicity, molecular weight, and water solubility were calculated using protparam and pepcalc tools. In addition, the probability of membrane binding potential and cellular localization of each CPP were estimated by Boman index using APD3 web server, D factor, and TMHMM web server. On the other hand, the immunogenicity, toxicity, allergenicity, hemolytic potency, and half-life of CPPs were predicted using various web servers. Finally, the tertiary structure and the helical wheel projection of some CPPs were predicted by PEP-FOLD3 and Heliquest web servers, respectively. These CPPs were divided into: a) CPP containing tumor homing motif (RGD) and/or tumor penetrating motif (RXXR); b) CPP with the highest Boman index; c) CPP with high half-life (~100 hour) in mammalian cells, and d) CPP with +5.00 net charge. Based on the results, we found a large number of novel CPPs with various features. Some of these CPPs possess tumor-specific motifs which can be evaluated in cancer therapy. Furthermore, the novel and potent CPPs derived from SARS-CoV-2 may be used alone or conjugated to some sequences such as nuclear localization sequence (NLS) for vaccine and drug delivery.

Anti-HIV-1 Activity of pepRF1, a Proteolysis-Resistant CXCR4 Antagonist Derived from Dengue Virus Capsid Protein

There is an urgent need for the development of new anti-HIV drugs that can complement existing medicines to be used against resistant strains. Here, we report the anti-HIV-1 peptide pepRF1, a human serum-resistant peptide derived from the Dengue virus capsid protein. In vitro, pepRF1 shows a 50% inhibitory concentration of 1.5 nM with a potential therapeutic window higher than 53 000. This peptide is specific for CXCR4-tropic strains, preventing viral entry into target cells by binding to the viral coreceptor CXCR4, acting as an antagonist of this receptor. pepRF1 is more effective than T20, the only peptide-based HIV-1 entry inhibitor approved, and excels in inhibiting a HIV-1 strain resistant to T20. Potentially, pepRF1 can be used alone or in combination with other anti-HIV drugs. Furthermore, one can also envisage its use as a novel therapeutic strategy for other CXCR4-related diseases.

Decoding the proteome of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) for cell-penetrating peptides involved in pathogenesis or applicable as drug delivery vectors

Synthetic or natural derived cell-penetrating peptides (CPPs) are vastly investigated as tools for the intracellular delivery of membrane-impermeable molecules. As viruses are intracellular obligate parasites, viral originated CPPs have been considered as suitable intracellular shuttling vectors for cargo transportation. A total of 310 CPPs were identified in the proteome of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Screening the proteome of the cause of COVID-19 reveals that SARS-CoV-2 CPPs (SCV2-CPPs) span the regions involved in replication, protein-nucleotide and protein-protein interaction, protein-metal ion interaction, and stabilization of homo/hetero-oligomers. However, to find the most appropriate peptides as drug delivery vectors, one might face several hurdles. Computational analyses showed that 94.3% of the identified SCV2-CPPs are non-toxins, and 38% are neither antigenic nor allergenic. Interestingly, 36.70% of SCV2-CPPs were resistant to all four groups of protease families. Nearly 1/3 of SCV2-CPPs had sufficient inherent or induced helix and sheet conformation leading to increased uptake efficiency. Heliquest lipid-binding discrimination factor revealed that 44.30% of the helical SCV2-CPPs are lipid-binding helices. Although Cys-rich derived CPPs of helicase (NSP13) can potentially fold into a cyclic conformation in endosomes with a higher rate of endosomal release, the most optimal SCV2-CPP candidates as vectors for drug delivery were SCV2-CPP118, SCV2-CPP119, SCV2-CPP122, and SCV2-CPP129 of NSP12 (RdRp). Ten experimentally validated viral-derived CPPs were also used as the positive control to check the scalability and reliability of our protocol in SCV2-CPP retrieval. Some peptides with a cell-penetration ability known as bioactive peptides are adopted as biotherapeutics themselves. Therefore, 59.60%, 29.63%, and 32.32% of SCV2-CPPs were identified as potential antibacterial, antiviral, and antifungals, respectively. While 63.64% of SCV2-CPPs had immuno-modulatory properties, 21.89% were recognized as anti-cancers. Conclusively, the workflow of this study provides a platform for profound screening of viral proteomes as a rich source of biotherapeutics or drug delivery carriers.

Cell-penetrating peptide-mediated cell entry of H5N1 highly pathogenic avian influenza virus

H5N1 highly pathogenic avian influenza virus (HPAIV) poses a huge threat to public health and the global economy. These viruses cause systemic infection in poultry and accidental human infection leads to severe pneumonia, associated with high mortality rates. The hemagglutinin (HA) of H5N1 HPAIV possesses multiple basic amino acids, as in the sequence RERRRKKR at the cleavage site; however, the role of this motif is not fully understood. Here, we showed that a 33-amino acid long peptide derived from HA of H5N1 HPAIV (HA314-46) has the potential to penetrate various cells and lung tissue through a sialic acid-independent endocytotic pathway. Mutant peptide analyses revealed that the cysteine residue at position 318 and multiple basic amino acids were essential for the cell-penetrating activity. Moreover, reassortant viruses possessing H5 HA could enter sialic acid-deficient cells, and virus internalisation was facilitated by cleavage with recombinant furin. Thus, our findings demonstrate that the HA314-46 motif exhibits cell-penetrating activity through a sialic acid-independent cell entry mechanism.

Enfuvirtide-Protoporphyrin IX Dual-Loaded Liposomes: In Vitro Evidence of Synergy against HIV-1 Entry into Cells

We have developed a nanocarrier consisting of large unilamellar vesicles (LUVs) for combined delivery of two human immunodeficiency virus type 1 (HIV-1) entry inhibitors, enfuvirtide (ENF) and protoporphyrin IX (PPIX). The intrinsic lipophilicity of ENF and PPIX, a fusion inhibitor and an attachment inhibitor, respectively, leads to their spontaneous incorporation into the lipid bilayer of the LUVs nanocarrier. Both entry inhibitors partition significantly toward LUVs composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and a 9:1 mixture of POPC:1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DPPE-PEG₂₀₀₀), representative of conventional and immune-evasive drug delivery formulations, respectively. These colocalize in the core of lipid membranes. Dual-loaded nanocarriers are monodispersed and retain the size distribution, thermotropic behavior, and surface charge of the unloaded form. Combination of the two entry inhibitors in the nanocarrier resulted in improved synergy against HIV-1 entry compared to combination in free form, strongly when immune-evasive formulations are used. We propose that the improved action of the entry inhibitors when loaded into the nanocarriers results from their slow release at the site of viral entry. Overall, liposomes remain largely unexplored platforms for combination of viral entry inhibitors, with potential for improvement of current antiretroviral therapy drug safety and application. Our work calls for a reappraisal of the potential of entry inhibitor combinations and delivery for clinical use in antiretroviral therapy.

Intracellular Delivery of DNA and Protein by a Novel Cell-Permeable Peptide Derived from DOT1L

Cellular uptake and intracellular release efficiency of biomacromolecules is low because of hurdles in the cell membrane that result in limited access to intra-cellular targets with few functional effects. Cell-penetrating peptides (CPPs) act as cargo delivery vehicles to promote therapeutic molecule translocation. Here, we describe the novel CPP-Dot1l that not only penetrates by itself, but also mediates cargo translocation in cultured cells, as confirmed by fluorescence microscopy and fluorescence spectrophotometry. We conducted cytotoxicity assays and safety evaluations, and determined peptide-membrane interactions to understand the possible pathway for cargo translocation. Additional nucleic acid and covalently conjugated green fluorescence protein (GFP) studies mediated by CPP-Dot1l were conducted to show functional delivery potential. Results indicate that CPP-Dot1l is a novel and effective CPP due to its good penetrating properties in different cell lines and its ability to enter cells in a concentration-dependent manner. Its penetration efficiency can be prompted by DMSO pretreatment. In addition, not only can it mediate plasmid delivery, but CPP-Dot1l can also deliver GFP protein into cytosol. In conclusion, the findings of this study showed CPP-Dot1l is an attractive pharmaceutical and biochemical tool for future drug, regenerative medicine, cell therapy, gene therapy, and gene editing-based therapy development.

Peptides with the multibasic cleavage site of the hemagglutinin from highly pathogenic influenza viruses act as cell-penetrating via binding to heparan sulfate and neuropilins

Cell-penetrating peptides (CPPs) show promise as an attractive delivery vehicle for therapeutic molecules-including nucleic acids, peptides, proteins, and even particulates-into several cell types. It is important to identify new CPPs and select the optimal CPP for each application, because CPPs differ in their internalized efficiency and internalization mechanisms. Here, we identified new CPPs derived from the peptides with the hemagglutinin cleavage site (pHACS) of highly pathogenic influenza viruses. We compared the potential of peptides from the pHACS of four subtypes of influenza A virus (H1, H3, H5, and H7) and an influenza B virus (H1-pHACS, H3-pHACS, H5-pHACS, H7-pHACS, and B-pHACS, respectively) to serve as CPPs. H5-pHACS and H7-pHACS, but not the other peptides, bound to mouse dendritic cells and human epithelial cells and were internalized efficiently into these cells. H5-pHACS and H7-pHACS required glycosaminoglycans, especially heparan sulfate and neuropilins, to bind to the cells. In addition, we designed a mutant H7-pHACS with superior cell-binding capability by changing a single amino acid. Furthermore, when conjugated with antigen, H5-pHACS and H7-pHACS induced antigen-specific antibody responses, demonstrating the usefulness of this antigen-delivery vehicle. Our results will improve our understanding of the mechanisms of CPPs and facilitate the development of novel drug-delivery vehicles designed to improve therapeutic efficacy.

Mechanisms of bacterial membrane permeabilization by crotalicidin (Ctn) and its fragment Ctn(15-34), antimicrobial peptides from rattlesnake venom

Crotalicidin (Ctn), a cathelicidin-related peptide from the venom of a South American rattlesnake, possesses potent antimicrobial, antitumor, and antifungal properties. Previously, we have shown that its C-terminal fragment, Ctn(15-34), retains the antimicrobial and antitumor activities but is less toxic to healthy cells and has improved serum stability. Here, we investigated the mechanisms of action of Ctn and Ctn(15-34) against Gram-negative bacteria. Both peptides were bactericidal, killing ∼90% of Escherichia coli and Pseudomonas aeruginosa cells within 90-120 and 5-30 min, respectively. Studies of ζ potential at the bacterial cell membrane suggested that both peptides accumulate at and neutralize negative charges on the bacterial surface. Flow cytometry experiments confirmed that both peptides permeabilize the bacterial cell membrane but suggested slightly different mechanisms of action. Ctn(15-34) permeabilized the membrane immediately upon addition to the cells, whereas Ctn had a lag phase before inducing membrane damage and exhibited more complex cell-killing activity, probably because of two different modes of membrane permeabilization. Using surface plasmon resonance and leakage assays with model vesicles, we confirmed that Ctn(15-34) binds to and disrupts lipid membranes and also observed that Ctn(15-34) has a preference for vesicles that mimic bacterial or tumor cell membranes. Atomic force microscopy visualized the effect of these peptides on bacterial cells, and confocal microscopy confirmed their localization on the bacterial surface. Our studies shed light onto the antimicrobial mechanisms of Ctn and Ctn(15-34), suggesting Ctn(15-34) as a promising lead for development as an antibacterial/antitumor agent.

A New Noncanonical Anionic Peptide That Translocates a Cellular Blood-Brain Barrier Model

The capacity to transport therapeutic molecules across the blood–brain barrier (BBB) represents a breakthrough in the development of tools for the treatment of many central nervous system (CNS)-associated diseases. The BBB, while being protective against infectious agents, hinders the brain uptake of many drugs. Hence, finding safe shuttles able to overcome the BBB is of utmost importance. Herein, we identify a new BBB-translocating peptide with unique properties. For years it was thought that cationic sequences were mandatory for a cell-penetrating peptide (CPP) to achieve cellular internalization. Despite being anionic at physiological pH, PepNeg (sequence (SGTQEEY) is an efficient BBB translocator that is able to carry a large cargo (27 kDa), while maintaining BBB integrity. In addition, PepNeg is able to use two distinct methods of translocation, energy-dependent and -independent, suggesting that direct penetration might occur when low concentrations of peptide are presented to cells. The discovery of this new anionic trans-BBB peptide allows the development of new delivery systems to the CNS and contributes to the need to rethink the role of electrostatic attraction in BBB-translocation.

Novel Peptides Derived from Dengue Virus Capsid Protein Translocate Reversibly the Blood-Brain Barrier through a Receptor-Free Mechanism

The delivery of therapeutic molecules to the central nervous system is hampered by poor delivery across the blood-brain barrier (BBB). Several strategies have been proposed to enhance transport into the brain, including invasive techniques and receptor-mediated transport (RMT). Both approaches have several drawbacks, such as BBB disruption, receptor saturation, and off-target effects, raising safety issues. Herein, we show that specific domains of Dengue virus type 2 capsid protein (DEN2C) can be used as trans-BBB peptide vectors. Their mechanism of translocation is receptor-independent and consistent with adsorptive-mediated transport (AMT). One peptide in particular, named PepH3, reaches equilibrium distribution concentrations across the BBB in less than 24 h in a cellular in vitro assay. Importantly, in vivo biodistribution data with radiolabeled peptide derivatives show high brain penetration. In addition, there is fast clearance from the brain and high levels of excretion, showing that PepH3 is a very good candidate to be used as a peptide shuttle taking cargo in and out of the brain.

Trends in the Binding of Cell Penetrating Peptides to siRNA: A Molecular Docking Study

The use of gene therapeutics, including short interfering RNA (siRNA), is limited by the lack of efficient delivery systems. An appealing approach to deliver gene therapeutics involves noncovalent complexation with cell penetrating peptides (CPPs) which are able to penetrate the cell membranes of mammals. Although a number of CPPs have been discovered, our understanding of their complexation and translocation of siRNA is as yet insufficient. Here, we report on computational studies comparing the binding affinities of CPPs with siRNA, considering a variety of CPPs. Specifically, seventeen CPPs from three different categories, cationic, amphipathic, and hydrophobic CPPs, were studied. Molecular mechanics were used to minimize structures, while molecular docking calculations were used to predict the orientation and favorability of sequentially binding multiple peptides to siRNA. Binding scores from docking calculations were highest for amphipathic peptides over cationic and hydrophobic peptides. Results indicate that initial complexation of peptides will likely occur along the major groove of the siRNA, driven by electrostatic interactions. Subsequent binding of CPPs is likely to occur in the minor groove and later on bind randomly, to siRNA or previously bound CPPs, through hydrophobic interactions. However, hydrophobic CPPs do not show this binding pattern. Ultimately binding yields a positively charged nanoparticle capable of noninvasive cellular import of therapeutic molecules.

Shifts in the fluorescence lifetime of EGFP during bacterial phagocytosis measured by phase-sensitive flow cytometry

Phase-sensitive flow cytometry (PSFC) is a technique in which fluorescence excited state decay times are measured as fluorescently labeled cells rapidly transit a finely focused, frequency-modulated laser beam. With PSFC the fluorescence lifetime is taken as a cytometric parameter to differentiate intracellular events that are challenging to distinguish with standard flow cytometry. For example PSFC can report changes in protein conformation, expression, interactions, and movement, as well as differences in intracellular microenvironments. This contribution focuses on the latter case by taking PSFC measurements of macrophage cells when inoculated with enhanced green fluorescent protein (EGFP)-expressing E. coli. During progressive internalization of EGFP-E. coli, fluorescence lifetimes were acquired and compared to control groups. It was hypothesized that fluorescence lifetimes would correlate well with phagocytosis because phagosomes become acidified and the average fluorescence lifetime of EGFP is known to be affected by pH. We confirmed that average EGFP lifetimes consistently decreased (3 to 2 ns) with inoculation time. The broad significance of this work is the demonstration of how high-throughput fluorescence lifetime measurements correlate well to changes that are not easily tracked by intensity-only cytometry, which is affected by heterogeneous protein expression, cell-to-cell differences in phagosome formation, and number of bacterium engulfed.

siRNA-cell-penetrating peptides complexes as a combinatorial therapy against chronic myeloid leukemia using BV173 cell line as model

Chronic myeloid leukemia (CML) is a myeloproliferative disorder caused by a single gene mutation, a reciprocal translocation that originates the Bcr-Abl gene with constitutive tyrosine kinase activity. As a monogenic disease, it is an optimum target for RNA silencing therapy. We developed a siRNA-based therapeutic approach in which the siRNA is delivered by pepM or pepR, two cell-penetrating peptides (CPPs) derived from the dengue virus capsid protein. These peptides have a dual role: siRNA delivery into cells and direct action as bioportides, i.e. intracellularly bioactive CPPs, targetting cancer-related signaling processes. Both pepM and pepR penetrate the positive Bcr-Abl⁺ Cell Line (BV173). Five in silico designed anti-Bcr-Abl siRNA were selected for in vitro analysis after thorough screening. The Bcr-Abl downregulation kinetics (48h to 168h) was followed by quantitative PCR. The bioportide action of the peptide vectors was evaluated by genome-wide microarray analysis and further validated by testing BV173 cell cycle and cell proliferation monitoring different genes involved in housekeeping/cell stress (RPL13A, HPRT1), cell proliferation (ki67), cell apoptosis (Caspase 3 and Caspase 9) and cell cycle steps (CDK2, CCDN2, CDKN1A). Assays with a commercial transfection agent were carried out for comparison purposes. Maximal Bcr-Abl gene knockdown was observed for one of the siRNA when delivered by pepM at 120h. Both pepM and pepR showed downregulation effects on proliferative CML-related signaling pathways having direct impact on BV173 cell cycle and proliferation, thus reinforcing the siRNA effect by acting as anticancer molecules. With this work we show the therapeutic potential of a CPP shuttle that combines intrinsic anticancer properties with the ability to deliver functional siRNA into CML cell models. By such combination, the pepM-siRNA conjugates lowered Bcr-Abl gene expression levels more extensively than conventional siRNA delivery technologies and perturbed leukemogenic cell homeostasis, hence revealing their potential as novel alternative scaffolds for CML therapy.

Cell Type Preference of a Novel Human Derived Cell-Permeable Peptide dNP2 and TAT in Murine Splenic Immune Cells

Cell-permeable peptides (CPPs) have been widely studied as an attractive drug delivery system to deliver therapeutic macromolecules such as DNA, RNA, and protein into cells. However, its clinical application is still limited and controversial due to the lack of a complete understanding of delivery efficiency in target cells. Previously we identified and characterized the novel and superior CPP, named dNP2, and here we comparatively analyzed intracellular delivery efficiency of dNP2 and TAT in various immune cells of mouse spleen to demonstrate their cell type preference. dNP2- or TAT-conjugated fluorescent proteins were most efficiently taken up by phagocytic cells such as dendritic cells and macrophages while little protein uptake was seen by lymphocytes including T cells, B cells, and NK cells. Interestingly CD8⁺ lymphoid dendritic cells and CD62L^loCD44^hi memory like T cell subsets showed significantly better uptake efficiency in vitro and in vivo relative to other dendritic cells or T cells, respectively. In addition, activated macrophages, T cells, and B cells took up the proteins more efficiently relative to when in the resting state. Importantly, only dNP2, not TAT, shows significant intracellular protein delivery efficiency in vivo. Collectively, this study provides important information regarding heterogeneous intracellular delivery efficiency of CPPs such as dNP2 and TAT with cell type preference in the spleen needed for its application in phagocytic cells or activated immune cells.

Cell-Penetrating Peptides Selectively Cross the Blood-Brain Barrier In Vivo

Cell-penetrating peptides (CPPs) are a group of peptides, which have the ability to cross cell membrane bilayers. CPPs themselves can exert biological activity and can be formed endogenously. Fragmentary studies demonstrate their ability to enhance transport of different cargoes across the blood-brain barrier (BBB). However, comparative, quantitative data on the BBB permeability of different CPPs are currently lacking. Therefore, the in vivo BBB transport characteristics of five chemically diverse CPPs, i.e. pVEC, SynB3, Tat 47-57, transportan 10 (TP10) and TP10-2, were determined. The results of the multiple time regression (MTR) analysis revealed that CPPs show divergent BBB influx properties: Tat 47-57, SynB3, and especially pVEC showed very high unidirectional influx rates of 4.73 μl/(g × min), 5.63 μl/(g × min) and 6.02 μl/(g × min), respectively, while the transportan analogs showed a negligible to low brain influx. Using capillary depletion, it was found that 80% of the influxed peptides effectively reached the brain parenchyma. Except for pVEC, all peptides showed a significant efflux out of the brain. Co-injection of pVEC with radioiodinated bovine serum albumin (BSA) did not enhance the brain influx of radiodionated BSA, indicating that pVEC does not itself significantly alter the BBB properties. A saturable mechanism could not be demonstrated by co-injecting an excess dose of non-radiolabeled CPP. No significant regional differences in brain influx were observed, with the exception for pVEC, for which the regional variations were only marginal. The observed BBB influx transport properties cannot be correlated with their cell-penetrating ability, and therefore, good CPP properties do not imply efficient brain influx.

Cancer cell surface induced peptide folding allows intracellular translocation of drug

Many lead molecules identified in drug discovery campaigns are eliminated from consideration due to poor solubility and low cell permeability. These orphaned molecules could have clinical value if solubilized and delivered properly. SVS-1 is a de novo designed peptide that preferentially folds at the surface of tumor cells, adopting a β-hairpin conformation that rapidly translocates into the cytoplasm, and ultimately nucleus, of cells. SVS-1 is stable in serum and small molecules attached to the peptide are effectively delivered to cancer cells via mechanisms involving physical translocation and, to a lesser extent, clathrin-dependent endocytosis. For example, ligating the model hydrophobic drug Paclitaxel (PTX) to SVS-1 improved its aqueous solubility by ~1000-fold and successfully delivered and released PTX to cancer cells in vitro and tumors in vivo without toxic adjuvants. These results suggest that SVS-1 can serve as a simple, effective delivery platform for molecules with poor solubility and permeability.

Receptors and routes of dengue virus entry into the host cells

Dengue is the most prevalent arthropod-borne viral disease, caused by dengue virus, a member of the Flaviviridae family. Its worldwide incidence is now a major health problem, with 2.5 billion people living in risk areas. In this review, we integrate the structural rearrangements of each viral protein and their functions in all the steps of virus entry into the host cells. We describe in detail the putative receptors and attachment factors in mammalian and mosquito cells, and the recognition of viral immunocomplexes via Fcγ receptor in immune cells. We also discuss that virus internalization might occur through distinct entry pathways, including clathrin-mediated or non-classical clathrin-independent endocytosis, depending on the host cell and virus serotype or strain. The implications of viral maturation in virus entry are also explored. Finally, we discuss the mechanisms of viral genome access to the cytoplasm. This includes the role of low pH-induced conformational changes in the envelope protein that mediate membrane fusion, and original insights raised by our recent work that supports the hypothesis that capsid protein would also be an active player in this process, acting on viral genome translocation into the cytoplasm.

siRNAs encapsulated in recombinant capsid protein derived from Dengue serotype 2 virus inhibits the four serotypes of the virus and proliferation of cancer cells

siRNA delivery potential of the Dengue virus capsid protein in cultured cells was recently reported, but target knockdown potential in the context of specific diseases has not been explored. In this study we have evaluated the utility of the protein as an siRNA carrier for anti Dengue viral and anti cancer applications using cell culture systems. We show that target specific siRNAs delivered using the capsid protein inhibit infection by the four serotypes of Dengue virus and proliferation of two cancer cell lines. Our data confirm the potential of the capsid for anti Dengue viral and anti cancer RNAi applications. In addition, we have optimized a fermentation strategy to improve the yield of Escherichia coli expressed D2C protein since the reported yields of E. coli expressed flaviviral capsid proteins are low.

Cell-penetrating peptides: A tool for effective delivery in gene-targeted therapies

The current landscapes of novel therapeutic approaches rely mostly on gene-targeted technologies, enabling to fight rare genomic diseases, from infections to cancer and hereditary diseases. Although, reaching the action-site for this novel treatments requires to deliver nucleic acids, or other macromolecules into cells, which may pose difficult tasks to pharmaceutical companies. To overcome this technological limitation, a wide variety of vectors have been developed in the past decades and have proven to be successful in delivering various therapeutics. Cell-penetrating peptides (CPP) have been one of the technologies widely studied and have been increasingly used to transport small RNA/DNA, plasmids, antibodies, and nanoparticles into cells. Despite the already proved huge potential that these peptide-based approaches may suggest, few advances have been put to pharmacological or clinical use. This review will describe the origin, development, and usage of CPP to deliver therapeutic agents into cells, with special emphasis on their current application to gene-therapies. Specifically, we will describe the current trials being conducted to treat cancer, gene disorders, and autoimmune diseases using CPP-based therapies. © 2014 IUBMB Life, 66(3):182-194, 2014.