Classes of Cell-Penetrating Peptides

Exploring Peptido-Nanocomposites in the Context of Amyloid Diseases

Therapeutic peptides are a major class of pharmaceutical drugs owing to their target-binding specificity as well as their versatility in inhibiting aberrant protein-protein interactions associated with human pathologies. Within the realm of amyloid diseases, the use of peptides and peptidomimetics tailor-designed to overcome amyloidogenesis has been an active research endeavor since the late 90s. In more recent years, incorporating nanoparticles for enhancing the biocirculation and delivery of peptide drugs has emerged as a frontier in nanomedicine, and nanoparticles have further demonstrated a potency against amyloid aggregation and cellular inflammation to rival strategies employing small molecules, peptides, and antibodies. Despite these efforts, however, a fundamental understanding of the chemistry, characteristics and function of peptido-nanocomposites is lacking, and a systematic analysis of such strategy for combating a range of amyloid pathogeneses is missing. Here we review the history, principles and evolving chemistry of constructing peptido-nanocomposites from bottom up and discuss their future application against amyloid diseases that debilitate a significant portion of the global population.

Peptide-Drug Conjugates: Design, Chemistry, and Drug Delivery System as a Novel Cancer Theranostic

The emergence of peptide-drug conjugates (PDCs) that utilize target-oriented peptide moieties as carriers of cytotoxic payloads, interconnected with various cleavable/noncleavable linkers, resulted in the key-foundation of the new era of targeted therapeutics. They are capable of retaining the integrity of conjugates in the blood circulatory system as well as releasing the drugs at the tumor microenvironment. Other valuable advantages are specificity and selectivity toward targeted-receptors, higher penetration ability, and drug-loading capacity, making them a suitable candidate to play their vital role as promising carrier agents. In this review, we summarized the types of cell-targeting (CTPs) and cell-penetrating peptides (CPPs) that have broad applications in the advancement of targeted drug-delivery systems (DDS). Moreover, the techniques to overcome the limitations of peptide-chemistry for their extensive implementation to construct the PDCs. Besides this, the diversified breakthrough of linker chemistry, and ample knowledge of various cytotoxic payloads used in PDCs in recent years, as well as the mechanism of action of PDCs was critically discussed. The principal aim is to provide scattered and diversified knowledge in one place and to help researchers understand the pinching knots in the science of PDC development, also their progression toward a bright future for PDCs as novel theranostics in clinical practice.

Peptide-conjugated antimiRs improve myotonic dystrophy type 1 phenotypes by promoting endogenous MBNL1 expression

Myotonic dystrophy type 1 (DM1) is a rare neuromuscular disease caused by a CTG repeat expansion in the DMPK gene that generates toxic RNA with a myriad of downstream alterations in RNA metabolism. A key consequence is the sequestration of alternative splicing regulatory proteins MBNL1/2 by expanded transcripts in the affected tissues. MBNL1/2 depletion interferes with a developmental alternative splicing switch that causes the expression of fetal isoforms in adults. Boosting the endogenous expression of MBNL proteins by inhibiting the natural translational repressors miR-23b and miR-218 has previously been shown to be a promising therapeutic approach. We designed antimiRs against both miRNAs with a phosphorodiamidate morpholino oligonucleotide (PMO) chemistry conjugated to cell-penetrating peptides (CPPs) to improve delivery to affected tissues. In DM1 cells, CPP-PMOs significantly increased MBNL1 levels. In some candidates, this was achieved using concentrations less than two orders of magnitude below the median toxic concentration, with up to 5.38-fold better therapeutic window than previous antagomiRs. In HSALR mice, intravenous injections of CPP-PMOs improve molecular, histopathological, and functional phenotypes, without signs of toxicity. Our findings place CPP-PMOs as promising antimiR candidates to overcome the treatment delivery challenge in DM1 therapy.

The light chain of tetanus toxin bound to arginine-rich cell-penetrating peptide inhibits cortical reaction in mouse oocytes

Introduction: Cortical reaction is a secretory process that occurs after a spermatozoon fuses with the oocyte, avoiding the fusion of additional sperm. During this exocytic event, the cortical granule membrane fuses with the oocyte plasma membrane. We have identified several molecular components involved in this process and confirmed that SNARE proteins regulate membrane fusion during cortical reaction in mouse oocytes. In those studies, we microinjected different nonpermeable reagents to demonstrate the participation of a specific protein in the cortical reaction. However, the microinjection technique has several limitations. In this work, we aimed to assess the potential of cell-penetrating peptides (CPP) as biotechnological tools for delivering molecules into oocytes, and to evaluate the functionality of the permeable tetanus toxin (bound to CPP sequence) during cortical reaction. Methods: Arginine-rich cell-penetrating peptides have demonstrated the optimal internalization of small molecules in mammalian cells. Two arginine-rich CPP were used in the present study. One, labeled with 5-carboxyfluorescein, to characterize the factors that can modulate its internalization, and the other, the permeable light chain of tetanus toxin, that cleaves the SNAREs VAMP1 and VAMP3 expressed in mouse oocytes. Results: Results showed that fluorescent CPP was internalized into the oocyte cytoplasm and that internalization was dependent on the concentration, time, temperature, and maturation stage of the oocyte. Using our functional assay to study cortical reaction, the light chain of tetanus toxin bound to arginine-rich cell-penetrating peptide inhibited cortical granules exocytosis. Discussion: Results obtained from the use of permeable peptides demonstrate that this CPP is a promising biotechnological tool to study functional macromolecules in mouse oocytes.

Enhancing the Effectiveness of Oligonucleotide Therapeutics Using Cell-Penetrating Peptide Conjugation, Chemical Modification, and Carrier-Based Delivery Strategies

Oligonucleotide-based therapies are a promising approach for treating a wide range of hard-to-treat diseases, particularly genetic and rare diseases. These therapies involve the use of short synthetic sequences of DNA or RNA that can modulate gene expression or inhibit proteins through various mechanisms. Despite the potential of these therapies, a significant barrier to their widespread use is the difficulty in ensuring their uptake by target cells/tissues. Strategies to overcome this challenge include cell-penetrating peptide conjugation, chemical modification, nanoparticle formulation, and the use of endogenous vesicles, spherical nucleic acids, and smart material-based delivery vehicles. This article provides an overview of these strategies and their potential for the efficient delivery of oligonucleotide drugs, as well as the safety and toxicity considerations, regulatory requirements, and challenges in translating these therapies from the laboratory to the clinic.

Choosing an Optimal Solvent Is Crucial for Obtaining Cell-Penetrating Peptide Nanoparticles with Desired Properties and High Activity in Nucleic Acid Delivery

Cell-penetrating peptides (CPPs) are highly promising transfection agents that can deliver various compounds into living cells, including nucleic acids (NAs). Positively charged CPPs can form non-covalent complexes with negatively charged NAs, enabling simple and time-efficient nanoparticle preparation. However, as CPPs have substantially different chemical and physical properties, their complexation with the cargo and characteristics of the resulting nanoparticles largely depends on the properties of the surrounding environment, i.e., solution. Here, we show that the solvent used for the initial dissolving of a CPP determines the properties of the resulting CPP particles formed in an aqueous solution, including the activity and toxicity of the CPP-NA complexes. Using different biophysical methods such as dynamic light scattering (DLS), atomic force microscopy (AFM), transmission and scanning electron microscopy (TEM and SEM), we show that PepFect14 (PF14), a cationic amphipathic CPP, forms spherical particles of uniform size when dissolved in organic solvents, such as ethanol and DMSO. Water-dissolved PF14, however, tends to form micelles and non-uniform aggregates. When dissolved in organic solvents, PF14 retains its α-helical conformation and biological activity in cell culture conditions without any increase in cytotoxicity. Altogether, our results indicate that by using a solvent that matches the chemical nature of the CPP, the properties of the peptide-cargo particles can be tuned in the desired way. This can be of critical importance for in vivo applications, where CPP particles that are too large, non-uniform, or prone to aggregation may induce severe consequences.

Recent Advances of Studies on Cell-Penetrating Peptides Based on Molecular Dynamics Simulations

With the ability to transport cargo molecules across cell membranes with low toxicity, cell-penetrating peptides (CPPs) have become promising candidates for next generation peptide-based drug delivery vectors. Over the past three decades since the first CPP was discovered, a great deal of work has been done on the cellular uptake mechanisms and the applications for the delivery of therapeutic molecules, and significant advances have been made. But so far, we still do not have a precise and unified understanding of the structure-activity relationship of the CPPs. Molecular dynamics (MD) simulations provide a method to reveal peptide-membrane interactions at the atomistic level and have become an effective complement to experiments. In this paper, we review the progress of the MD simulations on CPP-membrane interactions, including the computational methods and technical improvements in the MD simulations, the research achievements in the CPP internalization mechanism, CPP decoration and coupling, and the peptide-induced membrane reactions during the penetration process, as well as the comparison of simulated and experimental results.

Evaluation of the effect of a cell penetrating peptide (TAT) towards tailoring the targeting efficacy and tumor uptake of porphyrin

Cell penetrating peptides (CPPs) are known to possess a unique capacity to penetrate biological membranes and translocate various molecules into the cells. Therefore, porphyrin-CPP conjugates could be envisaged to boost the intracellular delivery of porphyrins thereby providing an improved tool for the development of agents for multi-modal applications for cancer management. Working in this direction, an unsymmetrically substituted porphyrin derivative was conjugated with a transactivating transcriptional activator peptide (TAT) and various in vitro and in vivo studies were carried out in order to study the effect of adding a CPP to the porphyrin derivative. MTT assay revealed the preferential light dependent toxicity of the porphyrin derivative which was further enhanced upon peptide conjugation. Fluorescence and flow cytometry studies revealed the relatively higher cellular internalization of the porphyrin-TAT conjugate in comparison with the porphyrin derivative. The elevated light dependent cell toxicity of the porphyrin-TAT conjugate along with its capability of generating cytotoxic singlet oxygen indicated the advantages of using the porphyrin-TAT conjugate for PDT applications. Also, porphyrin and the porphyrin-peptide conjugate were radiolabelled with 68Ga to investigate their possible potential as PET agents. In vivo biodistribution studies revealed a higher tumor uptake for the 68Ga-porphyrin-TAT conjugate (6.32 ± 1.24% IA per g) than for 68Ga-porphyrin (2.45 ± 0.88% IA per g) at 60 min post-administration. However, the observation of a higher non-target retention of the radiolabelled agents during in vivo studies might pose a limitation on their possible application in PET imaging.

A cationic lipid mediated CRISPR/Cas9 technique for the production of stable genome edited citrus plants

BACKGROUND

The genetic engineering of crops has enhanced productivity in the face of climate change and a growing global population by conferring desirable genetic traits, including the enhancement of biotic and abiotic stress tolerance, to improve agriculture. The clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) system has been found to be a promising technology for genomic editing. Protoplasts are often utilized for the development of genetically modified plants through in vitro integration of a recombinant DNA fragment into the plant genome. We targeted the citrus Nonexpressor of Pathogenesis-Related 3 (CsNPR3) gene, a negative regulator of systemic acquired resistance (SAR) that governs the proteasome-mediated degradation of NPR1 and developed a genome editing technique targeting citrus protoplast DNA to produce stable genome-edited citrus plants.

RESULTS

Here, we determined the best cationic lipid nanoparticles to deliver donor DNA and described a protocol using Lipofectamine™ LTX Reagent with PLUS Reagent to mediate DNA delivery into citrus protoplasts. A Cas9 construct containing a gRNA targeting the CsNPR3 gene was transfected into citrus protoplasts using the cationic lipid transfection agent Lipofectamine with or without polyethylene glycol (PEG, MW 6000). The optimal transfection efficiency for the encapsulation was 30% in Lipofectamine, 51% in Lipofectamine with PEG, and 2% with PEG only. Additionally, plasmid encapsulation in Lipofectamine resulted in the highest cell viability percentage (45%) compared with PEG. Nine edited plants were obtained and identified based on the T7EI assay and Sanger sequencing. The developed edited lines exhibited downregulation of CsNPR3 expression and upregulation of CsPR1.

CONCLUSIONS

Our results demonstrate that utilization of the cationic lipid-based transfection agent Lipofectamine is a viable option for the successful delivery of donor DNA and subsequent successful genome editing in citrus.

Divalent Metal Ions Boost Effect of Nucleic Acids Delivered by Cell-Penetrating Peptides

Cell-penetrating peptides (CPPs) are promising tools for the transfection of various substances, including nucleic acids, into cells. The aim of the current work was to search for novel safe and effective approaches for enhancing transfection efficiency of nanoparticles formed from CPP and splice-correcting oligonucleotide (SCO) without increasing the concentration of peptide. We analyzed the effect of inclusion of calcium and magnesium ions into nanoparticles on CPP-mediated transfection in cell culture. We also studied the mechanism of such transfection as well as its efficiency, applicability in case of different cell lines, nucleic acid types and peptides, and possible limitations. We discovered a strong positive effect of these ions on transfection efficiency of SCO, that translated to enhanced synthesis of functional reporter protein. We observed significant changes in intracellular distribution and trafficking of nanoparticles formed by the addition of the ions, without increasing cytotoxicity. We propose a novel strategy for preparing CPP-oligonucleotide nanoparticles with enhanced efficiency and, thus, higher therapeutic potential. Our discovery may be translated to primary cell cultures and, possibly, in vivo studies, with the aim of increasing CPP-mediated transfection efficiency and the likelihood of using CPPs in clinics.

Cell-Penetrating Peptides

In this introductory chapter, we first define cell-penetrating peptides (CPPs), give short overview of CPP history and discuss several aspects of CPP classification. Next section is devoted to the mechanism of CPP penetration into the cells, where direct and endocytic internalization of CPP is explained. Kinetics of internalization is discussed more extensively, since this topic is not discussed in other chapters of this book. At the end of this section some features of the thermodynamics of CPP interaction with the membrane is also presented. Finally, we present different cargoes that can be transferred into the cells by CPPs and briefly discuss the effect of cargo on the rate and efficiency of penetration into the cells.

Delivery of oligonucleotide-based therapeutics: challenges and opportunities

Nucleic acid-based therapeutics that regulate gene expression have been developed towards clinical use at a steady pace for several decades, but in recent years the field has been accelerating. To date, there are 11 marketed products based on antisense oligonucleotides, aptamers and small interfering RNAs, and many others are in the pipeline for both academia and industry. A major technology trigger for this development has been progress in oligonucleotide chemistry to improve the drug properties and reduce cost of goods, but the main hurdle for the application to a wider range of disorders is delivery to target tissues. The adoption of delivery technologies, such as conjugates or nanoparticles, has been a game changer for many therapeutic indications, but many others are still awaiting their eureka moment. Here, we cover the variety of methods developed to deliver nucleic acid-based therapeutics across biological barriers and the model systems used to test them. We discuss important safety considerations and regulatory requirements for synthetic oligonucleotide chemistries and the hurdles for translating laboratory breakthroughs to the clinic. Recent advances in the delivery of nucleic acid-based therapeutics and in the development of model systems, as well as safety considerations and regulatory requirements for synthetic oligonucleotide chemistries are discussed in this review on oligonucleotide-based therapeutics.

Cell-Penetrating Peptides Delivering siRNAs: An Overview

Cell-Penetrating Peptides (CPP) are valuable tools capable of crossing the plasma membrane to deliver therapeutic cargo inside cells. Small interfering RNAs (siRNA) are double-stranded RNA molecules capable of silencing the expression of a specific protein triggering the RNA interference (RNAi) pathway, but they are unable to cross the plasma membrane and have a short half-life in the bloodstream. In this overview, we assessed the many different approaches used and developed in the last two decades to deliver siRNA through the plasma membrane through different CPPs sorted according to three different loading strategies: covalent conjugation, complex formation, and CPP-decorated (functionalized) nanocomplexes. Each of these strategies has pros and cons, but it appears the latter two are the most commonly reported and emerging as the most promising strategies due to their simplicity of synthesis, use, and versatility. Recent progress with siRNA delivered by CPPs seems to focus on targeted delivery to reduce side effects and amount of drugs used, and it appears to be among the most promising use for CPPs in future clinical applications.

Synthetic strategies in construction of organic low molecular-weight carrier-drug conjugates

Inefficient transportation of polar metabolic inhibitors through cell membranes of eukaryotic and prokaryotic cells precludes their direct use as drug candidates in chemotherapy. One of the possible solutions to this problem is application of the 'Trojan horse' strategy, i.e. conjugation of an active substance with a molecular carrier of organic or inorganic nature, facilitating membrane penetration. In this work, the synthetic strategies used in rational design and preparation of conjugates of bioactive agents with three types of organic low molecular-weight carriers have been reviewed. These include iron-chelating agents, siderophores and cell-penetrating peptides. Moreover, a less known but very promising "molecular umbrella" conjugation strategy has been presented. Special attention has been paid on appropriate linking strategies, especially these allowing intracellular drug release after internalisation of a conjugate.

Hexokinase II-Derived Cell-Penetrating Peptide Mediates Delivery of MicroRNA Mimic for Cancer-Selective Cytotoxicity

Cancer cells are often characterized by elevated levels of mitochondrion-bound hexokinase II (HKII), which facilitates their survival, proliferation, and metastasis. Here, we have designed a cancer-selective cell-penetrating peptide (CPP) by covalently coupling a short penetration-accelerating sequence (PAS) to the mitochondrial membrane-binding N-terminal 15 amino acids of HKII (pHK). PAS-pHK mediates efficient cellular uptake and cytosolic delivery of a synthetic mimic of miR-126, a tumor suppressor miRNA downregulated in many malignancies. Following uptake by breast cancer MCF-7 cells, the CPP-miRNA conjugate is distributed throughout the cytosol and shows strong colocalization with mitochondria, where PAS-pHK induces depolarization of mitochondrial membrane potential, inhibition of metabolic activities, depletion of intracellular ATP levels, release of cytochrome c, and, finally, apoptosis. Concomitantly, the miR-126 cargo synergistically enhances the anticancer effects of PAS-pHK. Importantly, the PAS-pHK-miR-126 conjugate is not toxic to noncancerous MCF-10A and HEK-93 cells. Our results demonstrate the potential of PAS-pHK-mediated delivery of miRNA mimics as a novel cancer-selective therapeutic strategy.

Comparative Molecular Transporter Properties of Cyclic Peptides Containing Tryptophan and Arginine Residues Formed through Disulfide Cyclization

We have previously reported cyclic cell-penetrating peptides [WR]₅ and [WR]₄ as molecular transporters. To optimize further the utility of our developed peptides for targeted therapy in cancer cells using the redox condition, we designed a new generation of peptides and evaluated their cytotoxicity as well as uptake behavior against different cancer cell lines. Thus, cyclic [C(WR)_xC] and linear counterparts (C(WR)_xC), where x = 4–5, were synthesized using Fmoc/tBu solid-phase peptide synthesis, purified, and characterized. The compounds did not show any significant cytotoxicity (at 25 µM) against ovarian (SK-OV-3), leukemia (CCRF-CEM), gastric adenocarcinoma (CRL-1739), breast carcinoma (MDA-MB-231), and normal kidney (LLCPK) cells after 24 and 72 h incubation. Both cyclic [C(WR)₅C] and linear (C(WR)₅C) demonstrated comparable molecular transporter properties versus [WR]₅ in the delivery of a phosphopeptide (F′-GpYEEI) in CCRF-CEM cells. The uptake of F′-GpYEEI in the presence of 1,4-dithiothreitol (DTT) as the reducing agent was significantly improved in case of l(C(WR)₅C), while it was not changed by [C(WR)₅C]. Fluorescence microscopy also demonstrated a significant uptake of F′-GpYEEI in the presence of l(C(WR)₅C). Cyclic [C(WR)₅C] improved the uptake of the fluorescent-labeled anti-HIV drugs F′-d4T, F′-3TC, and F′-FTC by 3.0–4.9-fold. These data indicate that both [C(WR)₅C] and linear (C(WR)₅C) peptides can act as molecular transporters.

Combating Proteins with Proteins: Engineering Cell-Penetrating Peptide Antagonists of Amyloid-β Aggregation and Associated Neurotoxicity

A central event that underlies the etiology of Alzheimer's disease (AD) is the self-assembly of the amyloid-β (Aβ) peptide into aggregates termed amyloids. Increasing evidence implicates soluble prefibrillar Aβ oligomers in the neurodegeneration and synaptic dysfunction in AD. Recently we introduced a new class of highly promising antagonists of Aβ amyloidogenesis: designed cell-penetrating peptides (CPPs). These CPPs combine the attractive intrinsic properties of peptides (high target specificity and selectivity, biocompatibility, biodegradability, and ease and low cost of production) with potent therapeutic effects (inhibition of Aβ oligomerization, fiber formation, and neurotoxicity) and highly efficient delivery (to target cells and subcellular organelles).

Characterization of Peptide-Oligonucleotide Complexes Using Electron Microscopy, Dynamic Light Scattering, and Protease Resistance Assay

Cationic peptides designed for cellular delivery of nucleic acid molecules form noncovalent nanocomplexes with negatively charged oligonucleotides (ON). The electrostatically associated complexes are further compacted by hydrophobic interactions yielding nanoparticles (NP) of homogeneous shape and size that are efficiently taken up by cells. The shape and size of NP often correlate with the biological activity of delivered ON inside cells; and the stability and accessibility of NP in biological fluids govern its circulation in organism and the cellular uptake. Therefore, here we provide protocols for characterizing the shape and size and surface charge of peptide/ON NP by negative staining transmission electron microscopy (TEM) and dynamic light scattering (DLS) respectively, and analysis of NP stability against proteolytic degradation.

Designing Chimeric Molecules for Drug Discovery by Leveraging Chemical Biology

After the first seed concept introduced in the 18th century, different disciplines have attributed different names to dual-functional molecules depending on their application, including bioconjugates, bifunctional compounds, multitargeting molecules, chimeras, hybrids, engineered compounds. However, these engineered constructs share a general structure: a first component that targets a specific cell and a second component that exerts the pharmacological activity. A stable or cleavable linker connects the two modules of a chimera. Herein, we discuss the recent advances in the rapidly expanding field of chimeric molecules leveraging chemical biology concepts. This Perspective is focused on bifunctional compounds in which one component is a lead compound or a drug. In detail, we discuss chemical features of chimeric molecules and their use for targeted delivery and for target engagement studies.

Drug Delivery Strategies to Overcome the Blood-Brain Barrier (BBB)

The brain capillary endothelium serves both as an exchange site for gases and solutes between blood and brain and as a protective fence against neurotoxic compounds from the blood. While this "blood-brain barrier" (BBB) function protects the fragile environment in the brain, it also poses a tremendous challenge for the delivery of drug compounds to the brain parenchyma. Paracellular brain uptake of drug compounds is limited by the physical tightness of the endothelium, which is tightly sealed with junction complexes. Transcellular uptake of lipophilic drug compounds is limited by the activity of active efflux pumps in the luminal membrane. As a result, the majority of registered CNS drug compounds are small lipophilic compounds which are not efflux transporter substrates. Small molecule CNS drug development therefore focuses on identifying compounds with CNS target affinity and modifies these in order to optimize lipophilicity and decrease efflux pump interactions. Since efflux pump activity is limiting drug uptake, it has been investigated whether coadministration of drug compounds with efflux pump inhibitors could increase drug uptake. While the concept works to some extent, a lot of challenges have been encountered in terms of obtaining efficient inhibition while avoiding adverse effects.Some CNS drug compounds enter the brain via nutrient transport proteins, an example is the levodopa, a prodrug of Dopamine, which crosses the BBB via the large neutral amino acid transporter LAT1. While carrier-mediated transport of drug compounds may seem attractive, the development of drugs targeting transporters is very challenging, since the compounds should have a good fit to the binding site, while still maintaining their CNS target affinity.Receptor-mediated transport of drug compounds, especially biotherapeutics, conjugated to a receptor-binding ligand has shown some promise, although the amounts transported are rather low. This also holds true for drug-conjugation to cell-penetrating peptides. Due to the low uptake of biotherapeutics, barrier-breaching approaches such as mannitol injections and focused ultrasound have been employed with some success to patient groups with no other treatment options.

Disulphide-less crotamine is effective for formation of DNA-peptide complex but is unable to improve bovine embryo transfection

This study aimed to investigate the ability of disulphide-less crotamine (dLCr) to complex DNA and to evaluate whether the DNA-dLCr complex is capable of improving transfection in bovine embryos. Three experiments were performed to: (i) evaluate the formation and stability of the DNA-dLCr complex; (ii) assess the dLCr embryotoxicity by exposure of bovine embryos to dLCr; and (iii) assess the efficiency of bovine embryo transfection after microinjection of the DNA-dLCr complex or green fluorescent protein (GFP) plasmid alone (control). DNA complexation by dLCr after 30 min of incubation at 1:100 and 1:50 proportions presented higher efficiency (P < 0.05) than the two controls: native crotamine (NCr) 1:10 and lipofectamine. There was no difference between DNA-dLCr 1:25 and the controls. The DNA-dLCr complexation was evaluated at different proportions and times. In all, at least half of maximum complexation was achieved within the initial 30 min. No embryotoxicity of dLCr was verified after exposure of in vitro fertilized embryos to different concentrations of the peptide. The effectiveness of dLCr to improve exogenous gene expression was evaluated by microinjection of the DNA-dLCr complex into in vitro fertilized zygotes, followed by verification of both embryo development and GFP expression. From embryos microinjected with DNA only, 4.6% and 2.8% expressed the GFP transgene at day 5 and day 7, respectively. The DNA-dLCr complex did not increase the number of GFP-positive embryos. In conclusion, dLCr forms a complex with DNA and its application in in vitro culture is possible. However, the dLCr peptide sequence should be redesigned to improve GFP expression.

Design and in vitro delivery of HIV-1 multi-epitope DNA and peptide constructs using novel cell-penetrating peptides

OBJECTIVES

Developing an effective HIV vaccine that stimulates the humoral and cellular immune responses is still challenging because of the diversity of HIV-1 virus, polymorphism of human HLA and lack of a suitable delivery system.

RESULTS

Using bioinformatics tools, we designed a DNA construct encoding multiple epitopes. These epitopes were highly conserved within prevalent HIV-1 subtypes and interacted with prevalent class I and II HLAs in Iran and the world. The designed DNA construct included Nef_60-84, Nef_126-144, Vpr_34-47, Vpr_60-75, Gp160_30-53, Gp160_308-323 and P24_8-151 epitopes (i.e., nef-vpr-gp160-p24 DNA) which was cloned into pET-24a(+) and pEGFP-N1 vectors. The recombinant polyepitope peptide (rNef-Vpr-Gp160-P24; ~ 32 kDa) was successfully generated in E. coli expression system. The pEGFP-nef-vpr-gp160-p24 and rNef-Vpr-Gp160-P24 polyepitope peptide were delivered into HEK-293 T cells using cell-penetrating peptides (CPPs). The MPG and HR9 CPPs, as well as the novel LDP-NLS and CyLoP-1 CPPs, were utilized for DNA and peptide delivery into the cells, respectively. SEM results confirmed the formation of stable MPG/pEGFP-N1-nef-vpr-gp160-p24, HR9/pEGFP-N1-nef-vpr-gp160-p24, LDP-NLS/rNef-Vpr-Gp160-P24 and CyLoP-1/rNef-Vpr-Gp160-P24 nanoparticles with a diameter of < 200 nm through non-covalent bonds. MTT assay results indicated that these nanoparticles did not have any major toxicity in vitro. Fluorescence microscopy, flow cytometry and western blot data demonstrated that these CPPs could significantly deliver the DNA and peptide constructs into HEK-293 T cells.

CONCLUSION

The use of these CPPs can be considered as an approach in HIV vaccine development for in vitro and in vivo delivery of DNA and peptide constructs into mammalian cells.

Cyclic Cell-Penetrating Peptides as Efficient Intracellular Drug Delivery Tools

Cyclic cell-penetrating peptides are relatively a newer class of peptides that have a huge potential for the intracellular delivery of therapeutic agents aimed at treating challenging ailments like multidrug-resistant bacterial diseases, cancer, and HIV infection. Cell-penetrating peptides (CPPs) have been extensively explored as intracellular delivery vehicles; however, they have some inherent limitations like poor stability, endosomal entrapment, toxicity, and suboptimal cell penetration. Owing to their favorable properties that avoid these limitations, cyclic CPPs can provide a good alternative to linear CPPs. Several Reviews have been published in the past decade that cover CPPs and cyclic peptides independently. To the best of our knowledge, this is one of the first Reviews that covers cyclic CPPs comprehensively in the light of studies published so far. In this Review, we have detailed examples of cyclic CPPs, their structures, and cyclization strategies followed by a detailed account of their advantages over their linear counterparts. A hot area in cyclic CPPs is the exploration of cell-penetration mechanisms; this Review highlights this topic in detail. Finally, we will review the applications of cyclic CPPs, followed by conclusions and future prospects.

Global Analysis of Intercellular Homeodomain Protein Transfer

The homeodomain is found in hundreds of transcription factors that play roles in fate determination via cell-autonomous regulation of gene expression. However, some homeodomain-containing proteins (HPs) are thought to be secreted and penetrate neighboring cells to affect the recipient cell fate. To determine whether this is a general characteristic of HPs, we carried out a large-scale validation for intercellular transfer of HPs. Our screening reveals that intercellular transfer is a general feature of HPs, but it occurs in a cell-context-sensitive manner. We also found the secretion is not solely a function of the homeodomain, but it is supported by external motifs containing hydrophobic residues. Thus, mutations of hydrophobic residues of HPs abrogate secretion and consequently interfere with HP function in recipient cells. Collectively, our study proposes that HP transfer is an intercellular communication method that couples the functions of interacting cells.

Lee et al. evaluate capabilities of homeodomain proteins (HPs) for transfer between cells. They find that intercellular transfer is a general but cell-context-sensitive property of HP. Intercellular HP transfer can be an unconventional way for the cells to communicate with neighboring cells that associate structurally and functionally.

TAT-Modified ω-Conotoxin MVIIA for Crossing the Blood-Brain Barrier

As the first in a new class of non-opioid drugs, ω-Conotoxin MVIIA was approved for the management of severe chronic pains in patients who are unresponsive to opioid therapy. Unfortunately, clinical application of MVIIA is severely limited due to its poor ability to penetrate the blood-brain barrier (BBB), reaching the central nervous system (CNS). In the present study, we have attempted to increase MVIIA’s ability to cross the BBB via a fusion protein strategy. Our results showed that when the TAT-transducing domain was fused to the MVIIA C-terminal with a linker of varied numbers of glycine, the MVIIA-TAT fusion peptide exhibited remarkable ability to cross the bio-membranes. Most importantly, both intravenous and intranasal administrations of MVIIA-TAT in vivo showed therapeutic efficacy of analgesia. Compared to the analgesic effects of intracerebral administration of the nascent MVIIA, these systemic administrations of MVIIA-TAT require higher doses, but have much prolonged effects. Taken together, our results showed that TAT conjugation of MVIIA not only enables its peripheral administration, but also maintains its analgesic efficiency with a prolonged effective time window. Intranasal administration also rendered the MVIIA-TAT advantages of easy applications with potentially reduced side effects. Our results may present an alternative strategy to improve the CNS accessibility for neural active peptides.

Bioactive cell penetrating peptides and proteins in cancer: a bright future ahead

Peptides and proteins bear an extraordinary therapeutic potential to effectively and selectively target many components of cells currently considered undruggable. However, their intracellular delivery remains a critical challenge. Cell penetrating peptides and protein domains (CPPs) can be employed to translocate therapeutic polypeptides through the cellular membrane. Here, we describe examples of linear peptides and proteins, byciclic macropeptides and nanobodies that target key players in cancer development, with intrinsic and engineered cell penetrating ability. We also describe current solutions to the main challenges to their clinical viability.

Efficient Intracellular Delivery of Cell-Impermeable Cargo Molecules by Peptides Containing Tryptophan and Histidine

We have previously evaluated and reported numerous classes of linear and cyclic peptides containing hydrophobic and hydrophilic segments for intracellular delivery of multiple molecular cargos. Herein, a combination of histidine and tryptophan amino acids were designed and evaluated for their efficiency in intracellular delivery of cell-impermeable phosphopeptides and the anti-HIV drug, emtricitabine. Two new decapeptides, with linear and cyclic natures, both containing alternate tryptophan and histidine residues, were synthesized using Fmoc/tBu solid-phase chemistry. The peptides were characterized and purified by using matrix-assisted laser desorption/ionization (MALDI) spectroscopy and high-performance liquid chromatography (HPLC), respectively. These peptides did not show significant toxicity up to 100 µM in ovarian cancer (SK-OV-3) and leukemia cancer (CCRF-CEM) cells. Furthermore, the cellular uptake of a fluorescence (F’)-labeled cell-impermeable phosphopeptide (F’-GpYEEI) was enhanced in the presence of linear (WH)₅ and cyclic [WH]₅ by 2- and 8-fold, respectively, compared to the uptake of the phosphopeptide alone. The cellular uptake was not significantly changed in the presence of endocytosis inhibitors. Furthermore, the intracellular uptake of the fluorescently-labeled anti-HIV drug, emtricitabine (F’-FTC), by linear (WH)₅ and cyclic [WH]₅ in SK-OV-3 cancer cell lines was found to be enhanced by 3.5- and 9-fold, respectively, compared to that of the drug alone. Fluorescent uptake experiments confirmed the localization of F’-GpYEEI-loaded cyclic [WH]₅ intracellularly in the SK-OV-3 cancer cell line after 3 h of incubation. Thus, these data demonstrated that [WH]₅ containing tryptophan and histidine enhanced the cellular uptake of F’-GpYEEI and emtricitabine.

Arginine-Rich Cell-Penetrating Peptides Require Nucleolin and Cholesterol-Poor Subdomains for Translocation across Membranes

Proficient transport vectors called cell-penetrating peptides (CPPs) internalize into eukaryotic cells mostly via endocytic pathways and facilitate the uptake of various cargo molecules attached to them. However, some CPPs are able to induce disturbances in the plasma membrane and translocate through it seemingly in an energy-independent manner. For understanding this phenomenon, giant plasma membrane vesicles (GPMVs) derived from the cells are a beneficial model system, since GPMVs have a complex membrane composition comparable to the cells yet lack cellular energy-dependent mechanisms. We investigated the translocation of arginine-rich CPPs into GPMVs with different membrane compositions. Our results demonstrate that lower cholesterol content favors accumulation of nona-arginine and, additionally, sequestration of cholesterol increases the uptake of the CPPs in vesicles with higher cholesterol packing density. Furthermore, the proteins on the surface of vesicles are essential for the uptake of arginine-rich CPPs: downregulation of nucleolin decreases the accumulation and digestion of proteins on the membrane suppresses translocation even more efficiently.

Cell-penetrating peptide conjugates of gambogic acid enhance the antitumor effect on human bladder cancer EJ cells through ROS-mediated apoptosis

Background

Gambogic acid (GA) is the main active ingredient of resin gamboges and possesses anti-cancer activity toward various human cancer cells. However, clinical application of GA has been limited by its poor aqueous solubility and dose-limiting toxicities. Cell-penetrating peptides (CPPs) are widely used to deliver anti-cancer drugs into cancer cells and to enhance the water solubility of drugs.

Purpose

The object of this study was to synthesize peptide-drug conjugates in which the cell-penetrating peptide TAT (trans-activator of transcription) was conjugated to GA and evaluated the anti-cancer activity of this GA-CPP conjugate (GA-TAT) in EJ bladder cancer cells.

Methods

GA is built onto the TAT, and the GA-TAT conjugates are cleaved from the solid support and purified via HPLC. The equilibrium solubility of GA-TAT was measured using the shake-flask method. The effects of GA-TAT on EJ cell viability and proliferation were determined by MTT assay, Edu assay and colony formation assay, respectively. After treated with 1.0 μM GA-TAT for 24 h, the apoptosis rate of EJ cells were detected by Acridine orange/ethidium bromide (AO/EB) assay and flow cytometry assay. The proteins of caspase-3 (processing), caspase-9 (processing), Bcl-2 and Bax were analyzed by Western blotting, and the intracellular reactive oxygen species (ROS) production was evaluated by a reactive oxygen species assay.

Results

In contrast to free GA, the solubility of GA-TAT in water was significantly improved. Meanwhile, GA-TAT significantly increased EJ cellular uptake, toxicity and apoptosis. Mechanistic analysis revealed that GA-TAT enhanced the anti-cancer effect of GA against EJ cells through ROS-mediated apoptosis. The results were demonstrated that GA-TAT increased the ROS level in EJ cells, and N-acetyl-L-cysteine (NAC; a well-known ROS scavenger) inhibited GA-TAT-induced ROS generation and apoptosis. Additionally, GA-TAT activated caspase-3 and caspase-9 and down-regulated the Bcl-2/Bax ratio, but these effects were largely rescued by NAC.

Conclusion

GA-TAT has outstanding potential for promoting tumor apoptosis and exhibits promise for use in bladder cancer therapy.

Cell Penetrating Peptides as Molecular Carriers for Anti-Cancer Agents

Cell membranes with their selective permeability play important functions in the tight control of molecular exchanges between the cytosol and the extracellular environment as the intracellular membranes do within the internal compartments. For this reason the plasma membranes often represent a challenging obstacle to the intracellular delivery of many anti-cancer molecules. The active transport of drugs through such barrier often requires specific carriers able to cross the lipid bilayer. Cell penetrating peptides (CPPs) are generally 5–30 amino acids long which, for their ability to cross cell membranes, are widely used to deliver proteins, plasmid DNA, RNA, oligonucleotides, liposomes and anti-cancer drugs inside the cells. In this review, we describe the several types of CPPs, the chemical modifications to improve their cellular uptake, the different mechanisms to cross cell membranes and their biological properties upon conjugation with specific molecules. Special emphasis has been given to those with promising application in cancer therapy.

Cell-penetrating peptides (CPPs): From delivery of nucleic acids and antigens to transduction of engineered nucleases for application in transgenesis

Cell-penetrating peptides (CPPs) have been studied for their capacity to translocate across the lipid membrane of several cell types. In membrane translocation, these peptides can remarkably transport biologically active hydrophilic molecules, such as pharmaceuticals, nucleic acids (DNA and RNA) and even high-molecular-weight proteins, Fig. 3 into the cell cytoplasm and organelles. The development of CPPs as transduction agents includes the modification of gene and protein expression, the reprogramming and differentiation of induced pluripotent stem cells and the preparation of cellular vaccines. A relatively recent field of CPP application is the transduction of plasmid DNA vectors and CPP-fusion proteins to modify genomes and introduce new traits in cells and organisms. CPP-mediated transduction of components for genome editing is an advantageous alternative to viral DNA vectors. Engineered site-specific nucleases, such as Cre recombinase, ZFN, TALENs and CRISPR associated protein (Cas), have been coupled to CPPs, and the fused proteins have been used to permeate targeted cells and tissues. The functionally active fusion CPP-nucleases subsequently home to the nucleus, incise genomic DNA at specific sites and induce repair and recombination. This review has the objective of discussing CPPs and elucidating the prospective use of CPP-mediated transduction technology, particularly in genome modification and transgenesis.

Interactions between Membranes and "Metaphilic" Polypeptide Architectures with Diverse Side-Chain Populations

At physiological conditions, most proteins or peptides can fold into relatively stable structures that present on their molecular surfaces specific chemical patterns partially smeared out by thermal fluctuations. These nanoscopically defined patterns of charge, hydrogen bonding, and/or hydrophobicity, along with their elasticity and shape stability (folded proteins have Young's moduli of ∼1 × 10⁸ Pa), largely determine and limit the interactions of these molecules, such as molecular recognition and allosteric regulation. In this work, we show that the membrane-permeating activity of antimicrobial peptides (AMPs) and cell-penetrating peptides (CPPs) can be significantly enhanced using prototypical peptides with "molten" surfaces: metaphilic peptides with quasi-liquid surfaces and adaptable shapes. These metaphilic peptides have a bottlebrush-like architecture consisting of a rigid helical core decorated with mobile side chains that are terminated by cationic or hydrophobic groups. Computer simulations show that these flexible side chains can undergo significant rearrangement in response to different environments, giving rise to adaptable surface chemistry of the peptide. This quality makes it possible to control their hydrophobicity over a broad range while maintaining water solubility, unlike many AMPs and CPPs. Thus, we are able to show how the activity of these peptides is amplified by hydrophobicity and cationic charge, and rationalize these results using a quantitative mean-field theory. Computer simulations show that the shape-changing properties of the peptides and the resultant adaptive presentation of chemistry play a key enabling role in their interactions with membranes.

Peptides for nucleic acid delivery

Nucleic acids and their synthetic oligonucleotide (ON) analogs are a group of gene therapeutic compounds which hold enormous clinical potential. Despite their undoubted potential, clinical translation of these molecules, however, has been largely held back by their limited bioavailability in the target tissues/cells. To overcome this, many different drug delivery systems have been devised. Among others, short delivery peptides, called cell-penetrating peptides (CPPs), have been demonstrated to allow for efficient delivery of nucleic acids and their ON analogs, in both cell culture and animal models. In this review, we provide brief overview of the latest advances in nucleic acid delivery with CPPs, covering the two main vectorization strategies, covalent conjugation and nanoparticle formation-based approach. In conclusion, CPP-based drug delivery systems have the capacity to overcome the hurdle of delivery and thus have the potential to facilitate the clinical translation of nucleic acid-based therapeutics.

Relating structure and internalization for ROMP-based protein mimics

Elucidating the predominant cellular entry mechanism for protein transduction domains (PTDs) and their synthetic mimics (PTDMs) is a complicated problem that continues to be a significant source of debate in the literature. The PTDMs reported here provide a well-controlled platform to vary molecular composition for structure activity relationship studies to further our understanding of PTDs, their non-covalent association with cargo, and their cellular internalization pathways. Specifically, several guanidine rich homopolymers, along with an amphiphilic block copolymer were used to investigate the relationship between structure and internalization activity in HeLa cells, both alone and non-covalently complexed with EGFP by flow cytometery and confocal imaging. The findings indicate that while changing the amount of positive charge on our PTDMs does not seem to affect the endosomal uptake, the presence of hydrophobicity appears to be a critical factor for the polymers to enter cells either alone, or with associated cargo.

CPPsite 2.0: a repository of experimentally validated cell-penetrating peptides

CPPsite 2.0 (http://crdd.osdd.net/raghava/cppsite/) is an updated version of manually curated database (CPPsite) of cell-penetrating peptides (CPPs). The current version holds around 1850 peptide entries, which is nearly two times than the entries in the previous version. The updated data were curated from research papers and patents published in last three years. It was observed that most of the CPPs discovered/ tested, in last three years, have diverse chemical modifications (e.g. non-natural residues, linkers, lipid moieties, etc.). We have compiled this information on chemical modifications systematically in the updated version of the database. In order to understand the structure-function relationship of these peptides, we predicted tertiary structure of CPPs, possessing both modified and natural residues, using state-of-the-art techniques. CPPsite 2.0 also maintains information about model systems (in vitro/in vivo) used for CPP evaluation and different type of cargoes (e.g. nucleic acid, protein, nanoparticles, etc.) delivered by these peptides. In order to assist a wide range of users, we developed a user-friendly responsive website, with various tools, suitable for smartphone, tablet and desktop users. In conclusion, CPPsite 2.0 provides significant improvements over the previous version in terms of data content.