A novel CAV derived cell-penetrating peptide efficiently delivers exogenous molecules through caveolae-mediated endocytosis

Nature-inspired peptide of MtDef4 C-terminus tail enables protein delivery in mammalian cells

Cell-penetrating peptides show promise as versatile tools for intracellular delivery of therapeutic agents. Various peptides have originated from natural proteins with antimicrobial activity. We investigated the mammalian cell-penetrating properties of a 16-residue peptide with the sequence GRCRGFRRRCFCTTHC from the C-terminus tail of the Medicago truncatula defensin MtDef4. We evaluated the peptide's ability to penetrate multiple cell types. Our results demonstrate that the peptide efficiently penetrates mammalian cells within minutes and at a micromolar concentration. Moreover, upon N-terminal fusion to the fluorescent protein GFP, the peptide efficiently delivers GFP into the cells. Despite its remarkable cellular permeability, the peptide has only a minor effect on cellular viability, making it a promising candidate for developing a cell-penetrating peptide with potential therapeutic applications.

The Chemical Inhibitors of Endocytosis: From Mechanisms to Potential Clinical Applications

Endocytosis is one of the major ways cells communicate with their environment. This process is frequently hijacked by pathogens. Endocytosis also participates in the oncogenic transformation. Here, we review the approaches to inhibit endocytosis, discuss chemical inhibitors of this process, and discuss potential clinical applications of the endocytosis inhibitors.

Cell-Penetrating and Targeted Peptides Delivery Systems as Potential Pharmaceutical Carriers for Enhanced Delivery across the Blood-Brain Barrier (BBB)

Among the challenges to the 21st-century health care industry, one that demands special mention is the transport of drugs/active pharmaceutical agents across the blood-brain barrier (BBB). The epithelial-like tight junctions within the brain capillary endothelium hinder the uptake of most pharmaceutical agents. With an aim to understand more deeply the intricacies of cell-penetrating and targeted peptides as a powerful tool for desirable biological activity, we provide a critical review of both CPP and homing/targeted peptides as intracellular drug delivery agents, especially across the blood-brain barrier (BBB). Two main peptides have been discussed to understand intracellular drug delivery; first is the cell-penetrating peptides (CPPs) for the targeted delivery of compounds of interest (primarily peptides and nucleic acids) and second is the family of homing peptides, which specifically targets cells/tissues based on their overexpression of tumour-specific markers and are thus at the heart of cancer research. These small, amphipathic molecules demonstrate specific physical and chemical modifications aimed at increased ease of cellular internalisation. Because only a limited number of drug molecules can bypass the blood-brain barrier by free diffusion, it is essential to explore all aspects of CPPs that can be exploited for crossing this barrier. Considering siRNAs that can be designed against any target RNA, marking such molecules with high therapeutic potential, we present a synopsis of the studies on synthetic siRNA-based therapeutics using CPPs and homing peptides drugs that can emerge as potential drug-delivery systems as an upcoming requirement in the world of pharma- and nutraceuticals.

Passing of Nanocarriers across the Histohematic Barriers: Current Approaches for Tumor Theranostics

Over the past several decades, nanocarriers have demonstrated diagnostic and therapeutic (i.e., theranostic) potencies in translational oncology, and some agents have been further translated into clinical trials. However, the practical application of nanoparticle-based medicine in living organisms is limited by physiological barriers (blood-tissue barriers), which significantly hampers the transport of nanoparticles from the blood into the tumor tissue. This review focuses on several approaches that facilitate the translocation of nanoparticles across blood-tissue barriers (BTBs) to efficiently accumulate in the tumor. To overcome the challenge of BTBs, several methods have been proposed, including the functionalization of particle surfaces with cell-penetrating peptides (e.g., TAT, SynB1, penetratin, R8, RGD, angiopep-2), which increases the passing of particles across tissue barriers. Another promising strategy could be based either on the application of various chemical agents (e.g., efflux pump inhibitors, disruptors of tight junctions, etc.) or physical methods (e.g., magnetic field, electroporation, photoacoustic cavitation, etc.), which have been shown to further increase the permeability of barriers.

Membrane Internalization Mechanisms and Design Strategies of Arginine-Rich Cell-Penetrating Peptides

Cell-penetrating peptides (CPPs) have been discovered to deliver chemical drugs, nucleic acids, and macromolecules to permeate cell membranes, creating a novel route for exogenous substances to enter cells. Up until now, various sequence structures and fundamental action mechanisms of CPPs have been established. Among them, arginine-rich peptides with unique cell penetration properties have attracted substantial scientific attention. Due to the positively charged essential amino acids of the arginine-rich peptides, they can interact with negatively charged drug molecules and cell membranes through non-covalent interaction, including electrostatic interactions. Significantly, the sequence design and the penetrating mechanisms are critical. In this brief synopsis, we summarize the transmembrane processes and mechanisms of arginine-rich peptides; and outline the relationship between the function of arginine-rich peptides and the number of arginine residues, arginine optical isomers, primary sequence, secondary and ternary structures, etc. Taking advantage of the penetration ability, biomedical applications of arginine-rich peptides have been refreshed, including drug/RNA delivery systems, biosensors, and blood-brain barrier (BBB) penetration. Understanding the membrane internalization mechanisms and design strategies of CPPs will expand their potential applications in clinical trials.

An update on dual targeting strategy for cancer treatment

The key issue in the treatment of solid tumors is the lack of efficient strategies for the targeted delivery and accumulation of therapeutic cargoes in the tumor microenvironment (TME). Targeting approaches are designed for more efficient delivery of therapeutic agents to cancer cells while minimizing drug toxicity to normal cells and off-targeting effects, while maximizing the eradication of cancer cells. The highly complicated interrelationship between the physicochemical properties of nanoparticles, and the physiological and pathological barriers that are required to cross, dictates the need for the success of targeting strategies. Dual targeting is an approach that uses both purely biological strategies and physicochemical responsive smart delivery strategies to increase the accumulation of nanoparticles within the TME and improve targeting efficiency towards cancer cells. In both approaches, either one single ligand is used for targeting a single receptor on different cells, or two different ligands for targeting two different receptors on the same or different cells. Smart delivery strategies are able to respond to triggers that are typical of specific disease sites, such as pH, certain specific enzymes, or redox conditions. These strategies are expected to lead to more precise targeting and better accumulation of nano-therapeutics. This review describes the classification and principles of dual targeting approaches and critically reviews the efficiency of dual targeting strategies, and the rationale behind the choice of ligands. We focus on new approaches for smart drug delivery in which synthetic and/or biological moieties are attached to nanoparticles by TME-specific responsive linkers and advanced camouflaged nanoparticles.

Redesigning of Cell-Penetrating Peptides to Improve Their Efficacy as a Drug Delivery System

Cell-penetrating peptides (CPP) are promising tools for the transport of a broad range of compounds into cells. Since the discovery of the first members of this peptide family, many other peptides have been identified; nowadays, dozens of these peptides are known. These peptides sometimes have very different chemical–physical properties, but they have similar drawbacks; e.g., non-specific internalization, fast elimination from the body, intracellular/vesicular entrapment. Although our knowledge regarding the mechanism and structure–activity relationship of internalization is growing, the prediction and design of the cell-penetrating properties are challenging. In this review, we focus on the different modifications of well-known CPPs to avoid their drawbacks, as well as how these modifications may increase their internalization and/or change the mechanism of penetration.

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.

Peptide-Based Drug Delivery Systems

Peptide-based drug delivery systems have many advantages when compared to synthetic systems in that they have better biocompatibility, biochemical and biophysical properties, lack of toxicity, controlled molecular weight via solid phase synthesis and purification. Lysosomes, solid lipid nanoparticles, dendrimers, polymeric micelles can be applied by intravenous administration, however they are of artificial nature and thus may induce side effects and possess lack of ability to penetrate the blood-brain barrier. An analysis of nontoxic drug delivery systems and an establishment of prospective trends in the development of drug delivery systems was needed. This review paper summarizes data, mainly from the past 5 years, devoted to the use of peptide-based carriers for delivery of various toxic drugs, mostly anticancer or drugs with limiting bioavailability. Peptide-based drug delivery platforms are utilized as peptide–drug conjugates, injectable biodegradable particles and depots for delivering small molecule pharmaceutical substances (500 Da) and therapeutic proteins. Controlled drug delivery systems that can effectively deliver anticancer and peptide-based drugs leading to accelerated recovery without significant side effects are discussed. Moreover, cell penetrating peptides and their molecular mechanisms as targeting peptides, as well as stimuli responsive (enzyme-responsive and pH-responsive) peptides and peptide-based self-assembly scaffolds are also reviewed.

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.

Cell-penetrating peptides (CPPs): an overview of applications for improving the potential of nanotherapeutics

In the field of nanotherapeutics, gaining cellular entry into the cytoplasm of the target cell continues to be an ultimate challenge. There are many physicochemical factors such as charge, size and molecular weight of the molecules and delivery vehicles, which restrict their cellular entry. Hence, to dodge such situations, a class of short peptides called cell-penetrating peptides (CPPs) was brought into use. CPPs can effectively interact with the cell membrane and can assist in achieving the desired intracellular entry. Such strategy is majorly employed in the field of cancer therapy and diagnosis, but now it is also used for other purposes such as evaluation of atherosclerotic plaques, determination of thrombin levels and HIV therapy. Thus, the current review expounds on each of these mentioned aspects. Further, the review briefly summarizes the basic know-how of CPPs, their utility as therapeutic molecules, their use in cancer therapy, tumor imaging and their assistance to nanocarriers in improving their membrane penetrability. The review also discusses the challenges faced with CPPs pertaining to their stability and also mentions the strategies to overcome them. Thus, in a nutshell, this review will assist in understanding how CPPs can present novel possibilities for resolving the conventional issues faced with the present-day nanotherapeutics.

Dynamics of delivering aptamer targeted nano-drugs into cells

A targeted nano-drug delivery system has provided great potential and benefits to the diagnosis and therapy of cancers. Cell entry is a critical step for taking effect of the targeted nano-drug. In this report, the dynamics of delivering a single aptamer targeted polyamindoamine-camptothecin-AS1411 (PAMAM-CPT-AS1411) nano-drug into cells was investigated using a force tracing technique based on atomic force microscopy. The results show that the specific interaction of AS1411 and nucleolin, which is overexpressed on cancer cells, enhances the efficiency of the PAMAM-CPT-AS1411 cell entry. Moreover, the specific interaction induced receptor-mediated endocytosis prolongs the duration and decreases the speed of a single PAMAM-CPT-AS1411 cell entry, which is helpful to understand the targeted nano-drugs prolonging the therapeutic drug level. However, the required force for PAMAM-CPT-AS1411 cell entry is not changed. This report will provide a novel and potential method for achieving the precise dynamics of targeted nano-drug delivery.

Identification of a novel cell-penetrating peptide derived from the capsid protein of chicken anemia virus and its application in gene delivery

Cell membranes are a great obstacle for entrance of gene therapeutic agents. Cell-penetrating peptides (CPPs) have been proven as a promising gene delivery tool. However, the early TAT peptide derived from the HIV transcription activator protein has been proven that the sequence contains Furin protease cleaved motifs which limited the TAT application in delivery of exogenous active molecules. In the present study, through the bioinformatics and experimental approach, we have identified a novel CPP derived from the N terminus of VP1 protein of chicken anemia virus (CAV), designated as CVP1-N2, which is rich in arginine residues and contains α-helical structure. Then, the ability of CVP1-N2 cell penetrating was detected using confocal imaging and flow cytometry. FITC-labeled CVP1-N2 peptide could rapidly internalize into different types of live cells with dose dependence and without cytotoxic effects by MTT assay. Surprisingly, CVP1-N2 with a pattern of nuclear sub-location has shown the higher uptake efficiency than TAT. At 10, 1, and 0.1 μM, the mean relative internalization of CVP1-N2 was respectively 1.08-, 12-, and 75-fold higher than that of CVP1, as well as 1.6-, 56-, and 75-fold higher than that of TAT. Moreover, using endocytic inhibitors along with low-temperature stress validated that the CVP1-N2 internalization route is direct translocation pathway. Finally, the capacity of CVP1-N2 for delivery of gene into cells was determined, where it was able to carry red fluorescent protein (RFP) and apoptin genes into cells respectively and induce the apoptosis. All these data indicate that CVP1-N2 could be used as a novel gene delivery vehicle for gene therapy in the future. KEY POINTS: • 1CVP1-N2 was identified as a novel more efficient cell-penetrating peptide. • 2. CVP1-N2 localized to the nucleus through the direct transduction pathway. • 3. CVP1-N2 was able to deliver the apoptin gene into HCT116 cells and induce apoptosis.

Internalization mechanisms of cell-penetrating peptides

In today's modern era of medicine, macromolecular compounds such as proteins, peptides and nucleic acids are dethroning small molecules as leading therapeutics. Given their immense potential, they are highly sought after. However, their application is limited mostly due to their poor in vivo stability, limited cellular uptake and insufficient target specificity. Cell-penetrating peptides (CPPs) represent a major breakthrough for the transport of macromolecules. They have been shown to successfully deliver proteins, peptides, siRNAs and pDNA in different cell types. In general, CPPs are basic peptides with a positive charge at physiological pH. They are able to translocate membranes and gain entry to the cell interior. Nevertheless, the mechanism they use to enter cells still remains an unsolved piece of the puzzle. Endocytosis and direct penetration have been suggested as the two major mechanisms used for internalization, however, it is not all black and white in the nanoworld. Studies have shown that several CPPs are able to induce and shift between different uptake mechanisms depending on their concentration, cargo or the cell line used. This review will focus on the major internalization pathways CPPs exploit, their characteristics and regulation, as well as some of the factors that influence the cellular uptake mechanism.

Structural Requirement of Casein Peptides for Transcytosis through the Caco-2 Cell Monolayer: Hydrophobicity and Charge Property Affect the Transport Pathway and Efficiency

Casein is a rich source of bioactive peptides with complete amino acid composition. In this study, the casein peptides identified in our previous study with different hydrophobicities and charge properties were employed to investigate the transport efficiency via the transcytosis pathway across Caco-2 cell monolayers. Results revealed that the apparent permeability coefficient (P_app) values of transcytosis exhibited a linear correlation with a pI of positively charged peptides during bidirectional transport. A similar law was found as for the peptides with different hydrophobicities. The transcytosis route of Pep-II to Pep-VII appears to be the clathrin- and caveolin-independent transcytosis pathway as well as caveolae-mediated transcytosis pathway, showing a linear correlation with P_app values, respectively. Additionally, no direct correlation was shown between the hydrophobicity of peptides and clathrin-mediated transcytosis. Our results help to increase the bioaccessibility of peptide drugs across intestinal mucosa by developing strategies to alter the physicochemical properties without changing bioactivity.

Identification of nuclear localization signal and nuclear export signal of VP1 from the chicken anemia virus and effects on VP2 shuttling in cells

BACKGROUND

VP1 of the chicken anemia virus (CAV) is a structural protein that is required for virus encapsulation. VP1 proteins are present both in the nucleus and cytoplasm; however, the functional nuclear localization signal (NLS) and nuclear export signal (NES) of VP1 are still unknown. This study aimed to characterize the NLS and NES motifs of VP1 using bioinformatics methods and multiple-site fragment deletions, and investigate shuttling of VP2 from nucleus to cytoplasm by co-transfection with VP1.

METHODS

Two putative NLS motifs were predicted by the WoLF PSORT and NLStradamus programs from the amino acid sequence of VP1. Three NES motifs of VP1 were predicted by the NetNES 1.1 Server and ELM server programs. All mutants were created by multiple-site fragment deletion mutagenesis. VP1 and VP2 were co-expressed in cells using plasmid transfection.

RESULTS

A functional NLS motif was identified at amino acid residues 3 to 10 (RRARRPRG) of VP1. Critical amino acids 3 to 10 were significantly involved in nuclear import in cells and were evaluated using systematic deletion mutagenesis. Three NES motifs of VP1 were predicted by the NetNES 1.1 Server and ELM server programs. A functional NES was identified at amino acid residues 375 to 388 (ELDTNFFTLYVAQ). Leptomycin B (LMB) treatment demonstrated that VP1 export from nucleus to cytoplasm occurred through a chromosome region maintenance 1 (CRM1)-dependent pathway. With co-expression of VP1 and VP2 in cells, we observed that VP1 may transport VP2 from nucleus to cytoplasm.

CONCLUSION

Our data showed that VP1 of CAV contained functional NLS and NES motifs that modulated nuclear import and export through a CRM1-dependent pathway. Further, VP1 may play a role in the transport of VP2 from nucleus to cytoplasm.