Cell-Penetrating Peptides: a Useful Tool for the Delivery of Various Cargoes Into Cells

Cell-Penetrating Peptides: Promising Therapeutics and Drug-Delivery Systems for Neurodegenerative Diseases

Currently, one of the most significant and rapidly growing unmet medical challenges is the treatment of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). This challenge encompasses the imperative development of efficacious therapeutic agents and overcoming the intricacies of the blood-brain barrier for successful drug delivery. Here we focus on the delivery aspect with particular emphasis on cell-penetrating peptides (CPPs), widely used in basic and translational research as they enhance drug delivery to challenging targets such as tissue and cellular compartments and thus increase therapeutic efficacy. The combination of CPPs with nanomaterials such as nanoparticles (NPs) improves the performance, accuracy, and stability of drug delivery and enables higher drug loads. Our review presents and discusses research that utilizes CPPs, either alone or in conjugation with NPs, to mitigate the pathogenic effects of neurodegenerative diseases with particular reference to AD and PD.

An Endosomal Escape Trojan Horse Platform to Improve Cytosolic Delivery of Nucleic Acids

Endocytosis is a major bottleneck toward cytosolic delivery of nucleic acids, as the vast majority of nucleic acid drugs remain trapped within endosomes. Current trends to overcome endosomal entrapment and subsequent degradation provide varied success; however, active delivery agents such as cell-penetrating peptides have emerged as a prominent strategy to improve cytosolic delivery. Yet, these membrane-active agents have poor selectivity for endosomal membranes, leading to toxicity. A hallmark of endosomes is their acidic environment, which aids in degradation of foreign materials. Here, we develop a pH-triggered spherical nucleic acid that provides smart antisense oligonucleotide (ASO) release upon endosomal acidification and selective membrane disruption, termed DNA EndosomaL Escape Vehicle Response (DELVR). We anchor i-Motif DNA to a nanoparticle (AuNP), where the complement strand contains both an ASO sequence and a functionalized endosomal escape peptide (EEP). By orienting the EEP toward the AuNP core, the EEP is inactive until it is released through acidification-induced i-Motif folding. In this study, we characterize a small library of i-Motif duplexes to develop a structure-switching nucleic acid sequence triggered by endosomal acidification. We evaluate antisense efficacy using HIF1a, a hypoxic indicator upregulated in many cancers, and demonstrate dose-dependent activity through RT-qPCR. We show that DELVR significantly improves ASO efficacy in vitro. Finally, we use fluorescence lifetime imaging and activity measurement to show that DELVR benefits synergistically from nuclease- and pH-driven release strategies with increased ASO endosomal escape efficiency. Overall, this study develops a modular platform that improves the cytosolic delivery of nucleic acid therapeutics and offers key insights for overcoming intracellular barriers.

Unveiling Sticholysin II and plasmid DNA interaction: Implications for developing non-viral vectors

Non-viral gene delivery systems offer significant potential for gene therapy due to their versatility, safety, and cost advantages over viral vectors. However, their effectiveness can be hindered by the challenge of efficiently releasing the genetic cargo from endosomes to prevent degradation in lysosomes. To overcome this obstacle, functional components can be incorporated into these systems. Sticholysin II (StII) is one of the pore-forming proteins derived from the sea anemone Stichodactyla helianthus, known for its high ability to permeabilize cellular and model membranes. In this study, we aimed to investigate the interaction between StII, and a model plasmid (pDNA) as an initial step towards designing an improved vector with enhanced endosomal escape capability. The electrophoretic mobility shift assay (EMSA) confirmed the formation of complexes between StII and pDNA. Computational predictions identified specific residues involved in the StII-DNA interaction interface, highlighting the importance of electrostatic interactions and hydrogen bonds in mediating the binding. Atomic force microscopy (AFM) of StII-pDNA complexes revealed the presence of nodular fiber and toroid shapes. These complexes were found to have a predominantly micrometer size, as confirmed by dynamic light scattering (DLS) measurements. Despite increase in the overall charge, the complexes formed at the evaluated nitrogen-to-phosphorus (N/P) ratios still maintained a negative charge. Moreover, StII retained its pore-forming capacity regardless of its binding to the complexes. These findings suggest that the potential ability of StII to permeabilize endosomal membranes could be largely maintained when combined with nucleic acid delivery systems. Additionally, the still remaining negative charge of the complexes would enable the association of another positively charged component to compact pDNA. However, to minimize non-specific cytotoxic effects, it is advisable to explore methods to regulate the protein's activity in response to the microenvironment.

A Self-Regenerating Artificial Cell, that is One Step Closer to Living Cells: Challenges and Perspectives

Controllable, self-regenerating artificial cells (SRACs) can be a vital advancement in the field of synthetic biology, which seeks to create living cells by recombining various biological molecules in the lab. This represents, more importantly, the first step on a long journey toward creating reproductive cells from rather fragmentary biochemical mimics. However, it is still a difficult task to replicate the complex processes involved in cell regeneration, such as genetic material replication and cell membrane division, in artificially created spaces. This review highlights recent advances in the field of controllable, SRACs and the strategies to achieve the goal of creating such cells. Self-regenerating cells start by replicating DNA and transferring it to a location where proteins can be synthesized. Functional but essential proteins must be synthesized for sustained energy generation and survival needs and function in the same liposomal space. Finally, self-division and repeated cycling lead to autonomous, self-regenerating cells. The pursuit of controllable, SRACs will enable authors to make bold advances in understanding life at the cellular level, ultimately providing an opportunity to use this knowledge to understand the nature of life.

Research advances in peptide‒drug conjugates

Peptide‒drug conjugates (PDCs) are drug delivery systems consisting of a drug covalently coupled to a multifunctional peptide via a cleavable linker. As an emerging prodrug strategy, PDCs not only preserve the function and bioactivity of the peptides but also release the drugs responsively with the cleavable property of the linkers. Given the ability to significantly improve the circulation stability and targeting of drugs in vivo and reduce the toxic side effects of drugs, PDCs have already been extensively applied in drug delivery. Herein, we review the types and mechanisms of peptides, linkers and drugs used to construct PDCs, and summarize the clinical applications and challenges of PDC drugs.

Opportunities and challenges of protein-based targeted protein degradation

In the 20 years since the first report of a proteolysis targeting chimeric (PROTAC) molecule, targeted protein degradation (TPD) technologies have attempted to revolutionize the fields of chemical biology and biomedicine by providing exciting research opportunities and potential therapeutics. However, they primarily focus on the use of small molecules to recruit the ubiquitin proteasome system to mediate target protein degradation. This then limits protein targets to cytosolic domains with accessible and suitable small molecule binding pockets. In recent years, biologics such as proteins and nucleic acids have instead been used as binders for targeting proteins, thereby expanding the scope of TPD platforms to include secreted proteins, transmembrane proteins, and soluble but highly disordered intracellular proteins. This perspective summarizes the recent TPD platforms that utilize nanobodies, antibodies, and other proteins as binding moieties to deplete challenging targets, either through the ubiquitin proteasome system or the lysosomal degradation pathway. Importantly, the perspective also highlights opportunities and remaining challenges of current protein-based TPD technologies.

Boron-peptide conjugates with angiopep-2 for boron neutron capture therapy

Boron neutron capture therapy (BNCT) induces intracellular nuclear reaction to destroy cancer cells during thermal neutron irradiation. To selectively eliminate cancer cells but avoid harmful effects on normal tissues, novel boron-peptide conjugates with angiopep-2, namely ANG-B, were constructed and evaluated in preclinical settings. Boron-peptide conjugates were synthesized using solid-phase peptide synthesis, and the molecular mass was validated by mass spectrometry afterwards. Boron concentrations in 6 cancer cell lines and an intracranial glioma mouse model after treatments were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Phenylalanine (BPA) was tested in parallel for comparison. In vitro treatment with boron delivery peptides significantly increased boron uptake in cancer cells. BNCT with 5 mM ANG-B caused 86.5% ± 5.3% of clonogenic cell death, while BPA at the same concentration caused 73.3% ± 6.0% clonogenic cell death. The in vivo effect of ANG-B in an intracranial glioma mouse model was evaluated by PET/CT imaging at 31 days after BNCT. The mouse glioma tumours in the ANG-B-treated group were shrunk by 62.9% on average, while the BPA-treated tumours shrank by only 23.0%. Therefore, ANG-B is an efficient boron delivery agent, which has low cytotoxicity and high tumour-to-blood ratio. Based on these experimental results, we expected that ANG-B may leverage BNCT performance in clinical applications in future.

Design, preparation and characterization of octopus-like self-releasing intracellular protein transporter LEB5 based on Escherichia coli heat-labile enterotoxin

Despite the great potential of protein drugs as intracellular therapeutic agents, the unmet challenge in breaking through the cell membrane barrier and delivering them to intracellular targets remains. Therefore, developing safe and effective delivery vehicles is critical for fundamental biomedical research and clinical applications. In this study, we designed an octopus-like self-releasing intracellular protein transporter, the LEB5, based on the heat-labile enterotoxin. This carrier comprises five identical units, each of which has three main components: a linker, a self-releasing enzyme sensitivity loop, and the LTB transport domain. The LEB5 comprises five purified monomers that self-assemble to create a pentamer with ganglioside GM1 binding capacity. The fluorescent protein EGFP was used as a reporter system to identify the LEB5 features. The high-purity fusion protein ELEB monomer was produced from modified bacteria carrying pET24a(+)-eleb recombinant plasmids. EGFP protein could effectively detach from LEB5 by low dosage trypsin, according to electrophoresis analysis. The transmission electron microscopy results indicate that both LEB5 and ELEB5 pentamers exhibit a relatively regularly spherical shape, and the differential scanning calorimetry measurements further suggest that these proteins possess excellent thermal stability. Fluorescence microscopy revealed that LEB5 translocated EGFP into different cell types. Flow cytometry showed cellular differences in the transport capacity of LEB5. According to the confocal microscopy, fluorescence analysis and western blotting data, EGFP was transferred to the endoplasmic reticulum by the LEB5 carrier, detached from LEB5 by cleavage of the enzyme-sensitive loop, and released into the cytoplasm. Within the dosage range of LEB5 10-80 μg/mL, cell counting kit-8 assay revealed no significant changes in cell viability. These results demonstrated that LEB5 is a safe and effective intracellular self-releasing delivery vehicle capable of transporting and releasing protein medicines into cells.

Enhanced Transdermal Absorption of Hyaluronic Acid via Fusion with Pep-1 and a Hyaluronic Acid Binding Peptide

It is always big challenges for hyaluronic acid (HA) in transmembrane absorbing and efficient delivering to the skin. Pep-1, as one of the cell-penetrating peptides, has been documented to permeate various substances across cellular membranes without covalent binding. Here, a novel hyaluronic acid binding peptide (named HaBP) is designed, and then combined with Pep-1 to enhance the cell-penetrating efficiency of HA. The results of ELISA and immunofluorescence assay show that HaBP could bind with HA very well, and a combination of Pep-1 and HaBP could efficiently improve the transmembrane ability of HA. Furthermore, HA gradually enters the dermis from the surface of the skin in mice when it is administrated with both HaBP and Pep-1, while there are no obvious allergies or other adverse reactions during this process. This study finds a new method to promote the efficient transmembrane and transdermal absorption of HA, and throws some light on further research on the development of hyaluronic acid and its related cosmetics or drugs.

Medicinal Chemistry Strategies for the Development of Inhibitors Disrupting β-Catenin's Interactions with Its Nuclear Partners

Dysregulation of the Wnt/β-catenin signaling pathway is strongly associated with various aspects of cancer, including tumor initiation, proliferation, and metastasis as well as antitumor immunity, and presents a promising opportunity for cancer therapy. Wnt/β-catenin signaling activation increases nuclear dephosphorylated β-catenin levels, resulting in β-catenin binding to TCF and additional cotranscription factors, such as BCL9, CBP, and p300. Therefore, directly disrupting β-catenin's interactions with these nuclear partners holds promise for the effective and selective suppression of the aberrant activation of Wnt/β-catenin signaling. Herein, we summarize recent advances in biochemical techniques and medicinal chemistry strategies used to identify potent peptide-based and small-molecule inhibitors that directly disrupt β-catenin's interactions with its nuclear binding partners. We discuss the challenges involved in developing drug-like inhibitors that target the interactions of β-catenin and its nuclear binding partner into therapeutic agents.

Exploring Cell-Penetrating Peptides as Penetration Enhancers in Eye Drop Formulations Using a Reconstructed Human Corneal Epithelial Model

Ocular tissues function as biological barriers that hinder drug delivery, depending on the target tissue and route of administration, and must be overcome to achieve the desired therapeutic effect. Penetration enhancers have long been investigated to improve corneal drug penetration via eye drop instillation; however, further development is warranted owing to potential safety concerns. In the present study, we focused on cell-penetrating peptides (CPPs) as a penetration enhancer to address the requirements and explored CPP candidates suitable for corneal drug delivery. Using a reconstructed human corneal epithelial tissue model, LabCyte CORNEA-MODEL24 as an alternative to animal testing that is expected to have higher reproducibility than extracted eyeballs and octa-arginine (R8) as a representative model CPP with simple structure, we investigated the enhancement of 6-carboxyfluorescein (6-FAM) uptake by fluorescence imaging and the potential of eye irritation by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Also, surface plasmon resonance (SPR) evaluated the interaction between R8 and model compounds, suggesting that the stronger interaction could facilitate the corneal uptake of compounds. A comparative screening study of corneal uptake using various CPPs showed that the CPPs other than R8 also have the potential to enhance the corneal uptake of 6-FAM. In particular, penetratin (PNT) showed stronger fluorescence intensity. Through these findings, this manuscript provides beneficial information for the development of a novel corneal penetration enhancer with CPPs. In the future, it is expected that the basic findings with R8 will be verified to be applicable to other CPPs for development as penetration enhancers for eye drop formulation.

Delivery of superoxide dismutase by TAT and abalone peptides for the protection of skin cells against oxidative stress

Trichoderma reesei superoxide dismutase (TrSOD) is a well-characterized enzyme being stable between 30 and 90°C for 1 h with activity at pH between 2.6 and 9.0. This work aimed to clone, express, purify, and evaluate the protective effect antioxidant of this enzyme on skin cells when fused to transactivator of transcription (TAT) protein transduction domain of HIV-1 and abalone (Ab) peptides to allow cell penetration. TrSOD, TAT-TrSOD-Yfp (fused to yellow fluorescent protein), and Ab-TrSOD were expressed in E. coli and purified as soluble proteins. The cytotoxicity of the enzymes, at the concentrations of 1, 3, and 6 μmol/L, was evaluated for a period of 24 and 48 h of incubation, with no cytotoxic effect on 3T3 fibroblasts. The 3T3 cells were exposed to the oxidant agent tert-butyl hydroperoxide and evaluated for reactive oxygen species (ROS) generation, in the presence or not of the recombinant enzymes. TAT-TrSOD-Yfp was able to decrease the generation of ROS by 15% when used in the concentrations of 3 and 6 μmol/L in comparison to the control, but there was no difference in relation to the effect of TrSOD. Ab-TrSOD, when compared to TrSOD, promoted a decrease in the formation of ROS of 19% and 14% at the concentrations of 1 and 6 μmol/L, respectively, indicating that this recombinant form was more effective in reducing oxidative stress compared to SOD without the cell-penetrating peptide (CPP). Together, these results indicate that the fusion of SOD with these CPP increased the antioxidant capacity of fibroblasts, identified by the reduction in the generation of ROS. In addition, such molecules, in the concentrations initially used, were not toxic to the cells, opening perspectives for the development of products for antioxidant protection of the skin that may have therapeutic and cosmetic application.

Variants of the adeno-associated virus serotype 9 with enhanced penetration of the blood-brain barrier in rodents and primates

The development of gene therapies for the treatment of diseases of the central nervous system has been hindered by the limited availability of adeno-associated viruses (AAVs) that efficiently traverse the blood-brain barrier (BBB). Here, we report the rational design of AAV9 variants displaying cell-penetrating peptides on the viral capsid and the identification of two variants, AAV.CPP.16 and AAV.CPP.21, with improved transduction efficiencies of cells of the central nervous system on systemic delivery (6- to 249-fold across 4 mouse strains and 5-fold in cynomolgus macaques, with respect to the AAV9 parent vector). We also show that the neurotropism of AAV.CPP.16 is retained in young and adult macaques, that this variant displays enhanced transcytosis at the BBB as well as increased efficiency of cellular transduction relative to AAV9, and that it can be used to deliver antitumour payloads in a mouse model of glioblastoma. AAV capsids that can efficiently penetrate the BBB will facilitate the clinical translation of gene therapies aimed at the central nervous system.

Nanoparticle entry into cells; the cell biology weak link

Nanoparticles (NP) are attractive options for the therapeutic delivery of active pharmaceutical drugs, proteins and nucleic acids into cells, tissues and organs. Research into the development and application of NP most often starts with a diverse group of scientists, including chemists, bioengineers and material and pharmaceutical scientists, who design, fabricate and characterize NP in vitro (Stage 1). The next step (Stage 2) generally investigates cell toxicity as well as the processes by which NP bind, are internalized and deliver their cargo to appropriate model tissue culture cells. Subsequently, in Stage 3, selected NP are tested in animal systems, mostly mouse. Whereas the chemistry-based development and analysis in Stage 1 is increasingly sophisticated, the investigations in Stage 2 are not what could be regarded as 'state-of-the-art' for the cell biology field and the quality of research into NP interactions with cells is often sub-standard. In this review we describe our current understanding of the mechanisms by which particles gain entry into mammalian cells via endocytosis. We summarize the most important areas for concern, highlight some of the most common mis-conceptions, and identify areas where NP scientists could engage with trained cell biologists. Our survey of the different mechanisms of uptake into cells makes us suspect that claims for roles for caveolae, as well as macropinocytosis, in NP uptake into cells have been exaggerated, whereas phagocytosis has been under-appreciated.

Maximizing the recovery of the native p28 bacterial peptide with improved activity and maintained solubility and stability in Escherichia coli BL21 (DE3)

p28 is a natural bacterial product, which recently has attracted much attention as an efficient cell penetrating peptide (CPP) and a promising anticancer agent. Considering the interesting biological qualities of p28, maximizing its expression appears to be a prominent priority. The optimization of such bioprocesses might be facilitated by utilizing statistical approaches such as Design of Experiment (DoE). In this study, we aimed to maximize the expression of "biologically active" p28 in Escherichia coli BL21 (DE3) host by harnessing statistical tools and experimental methods. Using Minitab, Plackett-Burman and Box-Behnken Response Surface Methodology (RSM) designs were generated to optimize the conditions for the expression of p28. Each condition was experimentally investigated by assessing the biological activity of the purified p28 in the MCF-7 breast cancer cell line. Seven independent variables were investigated, and three of them including ethanol concentration, OD₆₀₀ of the culture at the time of induction, and the post-induction temperature were demonstrated to significantly affect the p28 expression in E. coli. The cytotoxicity, penetration efficiency, and total process time were measured as dependent variables. The optimized expression conditions were validated experimentally, and the final products were investigated in terms of expression yield, solubility, and stability in vitro. Following the optimization, an 8-fold increase of the concentration of p28 expression was observed. In this study, we suggest an optimized combination of effective factors to produce soluble p28 in the E. coli host, a protocol that results in the production of a significantly high amount of the biologically active peptide with retained solubility and stability.

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.

Synaptotagmin 1-mediated cell membrane penetration and dopamine release enhancement by latroeggtoxin-VI

Latroeggtoxin-VI (LETX-VI), a proteinaceous neurotoxin mined from the egg transcriptome of spider L. tredecimguttatus, was previously found to promote the release of dopamine from PC12 cells. However, the relevant molecular mechanism has not been fully clear. Here LETX-VI was demonstrated to rapidly penetrate the plasma membrane of PC12 cells via the vesicle exocytosis/endocytosis cycle, during which vesicular transmembrane protein synaptotagmin 1 (Syt1) functions as a receptor, with its vesicle luminal domain interacting with the C-terminal region of LETX-VI. The C-terminal sequence of LETX-VI is the functional region for both entering cells and promoting dopamine release. After gaining entry into the PC12 cells, LETX-VI down-regulated the phosphorylation levels of Syt1 at T201 and T195, thereby facilitating vesicle fusion with plasma membrane and thus promoting dopamine release. The relevant mechanism analysis indicated that LETX-VI has a protein phosphatase 2A (PP2A) activator activity. The present work has not only probed into the Syt1-mediated action mechanism of LETX-VI, but also revealed the structure-function relationship of the toxin, thus suggesting its potential applications in the drug transmembrane delivery and treatment of the diseases related to dopamine release and PP2A activity deficiency.

The Potential of Cell-Penetrating Peptides for mRNA Delivery to Cancer Cells

In vitro transcribed mRNA for the synthesis of any given protein has shown great potential in cancer gene therapy, especially in cancer vaccines for immunotherapy. To overcome physiological barriers, such as rapid degradation by enzymatic attack and poor cellular uptake due to their large size and hydrophilic properties, many delivery carriers for mRNAs are being investigated for improving the bioavailability of mRNA. Recently, cell-penetrating peptides (CPPs) have received attention as promising tools for gene delivery. In terms of their biocompatibility and the ability to target specific cells with the versatility of peptide sequences, they may provide clues to address the challenges of conventional delivery systems for cancer mRNA delivery. In this study, optimal conditions for the CPP/mRNA complexes were identified in terms of complexation capacity and N/P ratio, and protection against RNase was confirmed. When cancer cells were treated at a concentration of 6.8 nM, which could deliver the highest amount of mRNA without toxicity, the amphipathic CPP/mRNA complexes with a size less than 200 nm showed high cellular uptake and protein expression. With advances in our understanding of CPPs, CPPs designed to target tumor tissues will be promising for use in developing a new class of mRNA delivery vehicles in cancer therapy.

Anti-cancer peptides: their current trends in the development of peptide-based therapy and anti-tumor drugs

Human cancer remains a cause of high mortality throughout the world. The conventional methods and therapies currently employed for treatment are followed by moderate-to-severe side effects. They have not generated curative results due to the ineffectiveness of treatments. Besides, the associated high costs, technical requirements, and cytotoxicity further characterize their limitations. Due to relatively higher presidencies, bioactive peptides with anti-cancer attributes have recently become treatment choices within the therapeutic arsenal. The peptides act as potential anti-cancer agents explicitly targeting tumor cells while being less toxic to normal cells. The anti-cancer peptides are isolated from various natural sources, exhibit high selectivity and high penetration efficiency, and could be quickly restructured. The therapeutic benefits of compatible anti-cancer peptides have contributed to the significant expansion of cancer treatment; albeit, the mechanisms by which bioactive peptides inhibit the proliferation of tumor cells remain unclear. This review will provide a framework for assessing anti-cancer peptides' structural and functional aspects. It shall provide appropriate information on their mode of action to support and strengthen efforts to improve cancer prevention. The article will mention the therapeutic health benefits of anti-cancer peptides. Their importance in clinical studies is elaborated for reducing cancer incidences and developing sustainable treatment models.

Design and use of chimeric peptides in a new non-destructive ecological process applied to the extraction of all trans/9-cis β-carotene isomers from Dunaliella salina

Recently, β-carotene has gained tremendous importance as a bioactive molecule due to the growing awareness of the harmful effects of synthetic products. β-carotene is a high-value natural pigment that has the highest demand in the global carotenoid market owing to its proven antioxidant properties relevant for several diseases. To date, Dunaliella salina is the most important producer of natural β-carotene and is the subject of important industrial efforts. However, the extraction of β-carotene remains challenging since all the proposed techniques present a risk of product contamination or loss of quality due to solvent residuals and low yields. The purpose of this study was to set up a green, ecological, and innovative process of extraction of the two major β-carotene isomers from the halophilic microalgae Dunaliella salina. Based on molecular modeling, docking, and drug design, we conceived and synthesized two chimeric peptides (PP2, PP3) targeting specifically the two major isomers: all-trans or 9-cis β-carotene. The experimental protocol used in this study demonstrated the ability and the efficacy of those two peptides to cross the cell membrane and bind with high affinity to β-carotene isomers and exclude them toward the extracellular medium while preserving the integrity of living cells. Interestingly, the tested peptides (PP2, PP3) exhibit significant β-carotene extraction yields 58% and 34%, respectively, from the total of the β-carotene in microalgae cells. In addition to its simplicity, this process is fast, independent of the source of the β-carotene, and selective. These results would allow us to set up a green, ecological, and very profitable process of extraction from microalgae containing high amounts of β-carotene. Our innovative approach is highly promising for the extraction of Dunaliella salina biomass on an industrial scale.

How Cargo Identity Alters the Uptake of Cell-Penetrating Peptide (CPP)/Cargo Complexes: A Study on the Effect of Net Cargo Charge and Length

Cell-penetrating peptides (CPPs) have emerged as a powerful tool for the delivery of otherwise impermeable cargoes into intact cells. Recent efforts to improve the delivery capability of peptides have mainly focused on the identity of the CPP; however, there is evidence that the identity of the cargo itself affects the uptake. The goal of this work was to investigate how the characteristics of a peptide cargo, including net charge and length, either enhance or diminish the internalization efficiency of the CPP/cargo complex. A small library of CPP/cargo complexes were synthesized consisting of structured and unstructured CPPs with cargoes of net positive, negative, or neutral charge and lengths of 4 or 8 amino acids. Cargoes with a net positive charge were found to enhance the overall uptake of the complexes while net neutral and negatively charged cargoes diminished uptake. Conversely, the net length of the cargo had no significant effect on uptake of the CPP/cargo complexes. Microcopy images confirmed the increased uptake of the positively charged cargoes; however, an increase in punctate regions with the addition of a cargo was also observed. The effects of the net positively charged cargoes were confirmed with both structured and unstructured CPPs, which demonstrated similar trends of an increase in uptake with the addition of positively charged residues. These findings demonstrate that the net charge of cargoes impacts the uptake of the complex, which can be considered in the future when designing peptide-based reporters or therapeutics.

Genome editing via non-viral delivery platforms: current progress in personalized cancer therapy

Cancer is a severe disease that substantially jeopardizes global health. Although considerable efforts have been made to discover effective anti-cancer therapeutics, the cancer incidence and mortality are still growing. The personalized anti-cancer therapies present themselves as a promising solution for the dilemma because they could precisely destroy or fix the cancer targets based on the comprehensive genomic analyses. In addition, genome editing is an ideal way to implement personalized anti-cancer therapy because it allows the direct modification of pro-tumor genes as well as the generation of personalized anti-tumor immune cells. Furthermore, non-viral delivery system could effectively transport genome editing tools (GETs) into the cell nucleus with an appreciable safety profile. In this manuscript, the important attributes and recent progress of GETs will be discussed. Besides, the laboratory and clinical investigations that seek for the possibility of combining non-viral delivery systems with GETs for the treatment of cancer will be assessed in the scope of personalized therapy.

Nanomaterial Probes for Nuclear Imaging

Nuclear imaging is a powerful non-invasive imaging technique that is rapidly developing in medical theranostics. Nuclear imaging requires radiolabeling isotopes for non-invasive imaging through the radioactive decay emission of the radionuclide. Nuclear imaging probes, commonly known as radiotracers, are radioisotope-labeled small molecules. Nanomaterials have shown potential as nuclear imaging probes for theranostic applications. By modifying the surface of nanomaterials, multifunctional radio-labeled nanomaterials can be obtained for in vivo biodistribution and targeting in initial animal imaging studies. Various surface modification strategies have been developed, and targeting moieties have been attached to the nanomaterials to render biocompatibility and enable specific targeting. Through integration of complementary imaging probes to a single nanoparticulate, multimodal molecular imaging can be performed as images with high sensitivity, resolution, and specificity. In this review, nanomaterial nuclear imaging probes including inorganic nanomaterials such as quantum dots (QDs), organic nanomaterials such as liposomes, and exosomes are summarized. These new developments in nanomaterials are expected to introduce a paradigm shift in nuclear imaging, thereby creating new opportunities for theranostic medical imaging tools.

Recent applications of cell-penetrating peptide guidance of nanosystems in breast and prostate cancer (Review)

Cell-penetrating peptides (CPPs) are small peptides from natural sources or designed from other protein sequences that can penetrate cell membranes. This property has been used in biomedicine to add them to biomolecules to improve their capacity for cell internalization and as a guidance tool for specific cell types. CPPs have been shown to enhance cellular uptake in vitro and in vivo, improving the efficacy of anticancer drugs such as doxorubicin and paclitaxel, while also limiting their cytotoxic effects on healthy cells and tissues. The current study reviews the internalization and major therapeutic results achieved from the functionalization of nanosystems with CPPs for guidance into breast and prostate cancer cells in vitro and in vivo. In addition, the practical results obtained are specifically discussed for use as a starting point for scientists looking to begin research in this field.

Molecular dynamics study of the internalization of cell-penetrating peptides containing unnatural amino acids across membranes

Peptide-based delivery systems that deliver target molecules into cells have been gaining traction. These systems need cell-penetrating peptides (CPPs), which have the remarkable ability to penetrate into biological membranes and help internalize different cargoes into cells through the cell membranes. The molecular internalization mechanism and structure-function relationships of CPPs are not clear, although the incorporation of nonproteinogenic amino acids such as α-aminoisobutyric acid (Aib) has been reported to increase their helicity, biostability and penetration efficiencies. Here, we used molecular dynamics to study two Aib-containing CPPs, poly(LysAibAla)₃ (KAibA) and poly(LysAibGly)₃ (KAibG), that previously showed high cell internalization efficiency. KAibA and KAibG displayed the lowest internalization energies among the studied CPPs, showing distinct internalization mechanisms depending on the lipid composition of the model membranes. The presence of Aib residues allows these CPPs to adopt amphipathic folding to efficiently penetrate through the membranes. Elucidating how Aib incorporation affects CPP-membrane binding and interactions is beneficial for the design of CPPs for efficient intracellular delivery.

[(WR)8WKβA]-Doxorubicin Conjugate: A Delivery System to Overcome Multi-Drug Resistance against Doxorubicin

Doxorubicin (Dox) is an anthracycline chemotherapeutic agent used to treat breast, leukemia, and lymphoma malignancies. However, cardiotoxicity and inherent acquired resistance are major drawbacks, limiting its clinical application. We have previously shown that cyclic peptide [WR]₉ containing alternate tryptophan (W) and arginine (R) residues acts as an efficient molecular transporter. An amphiphilic cyclic peptide containing a lysine (K) residue and alternative W and R was conjugated through a free side chain amino group with Dox via a glutarate linker to afford [(WR)₈WKβA]-Dox conjugate. Antiproliferative assays were performed in different cancer cell lines using the conjugate and the corresponding physical mixture of the peptide and Dox to evaluate the effectiveness of synthesized conjugate compared to the parent drug alone. [(WR)₈WKβA]-Dox conjugate showed higher antiproliferative activity at 10 µM and 5 µM than Dox alone at 5 μM. The conjugate inhibited the cell viability of ovarian adenocarcinoma (SK-OV-3) by 59% and the triple-negative breast cancer cells MDA-MB-231 and MCF-7 by 71% and 77%, respectively, at a concentration of 5 μM after 72 h of incubation. In contrast, Dox inhibited the proliferation of SK-OV-3, MDA-MB-231, and MCF-7 by 35%, 63%, and 57%, respectively. Furthermore, [(WR)₈WKβA]-Dox conjugate (5 µM) inhibited the cell viability of Dox-resistant cells (MES-SA/MX2) by 92%, while the viability of cells incubated with free Dox was only 15% at 5 μM. Confocal microscopy images confirmed the ability of both Dox conjugate and the physical mixture of the peptide with the drug to deliver Dox through an endocytosis-independent pathway, as the uptake was not inhibited in the presence of endocytosis inhibitors. The stability of Dox conjugate was observed at different time intervals using analytical HPLC when the conjugate was incubated with 25% human serum. Half-life (t_1/2) for [(WR)₈WKβA]-Dox conjugate was (∼6 h), and more than 80% of the conjugate was degraded at 12 h. The release of free Dox was assessed intracellularly using the CCRF-CEM cell line. The experiment demonstrated that approximately 100% of free Dox was released from the conjugate intracellularly within 72 h. These data confirm the ability of the cyclic cell-penetrating peptide containing tryptophan and arginine residues as an efficient tool for delivery of Dox and for overcoming resistance to it.

[(WR)8WKβA]-Doxorubicin Conjugate: A Delivery System to Overcome Multi-Drug Resistance against Doxorubicin

Doxorubicin (Dox) is an anthracycline chemotherapeutic agent used to treat breast, leukemia, and lymphoma malignancies. However, cardiotoxicity and inherent acquired resistance are major drawbacks, limiting its clinical application. We have previously shown that cyclic peptide [WR]9 containing alternate tryptophan (W) and arginine (R) residues acts as an efficient molecular transporter. An amphiphilic cyclic peptide containing a lysine (K) residue and alternative W and R was conjugated through a free side chain amino group with Dox via a glutarate linker to afford [(WR)8WKβA]-Dox conjugate. Antiproliferative assays were performed in different cancer cell lines using the conjugate and the corresponding physical mixture of the peptide and Dox to evaluate the effectiveness of synthesized conjugate compared to the parent drug alone. [(WR)8WKβA]-Dox conjugate showed higher antiproliferative activity at 10 µM and 5 µM than Dox alone at 5 μM. The conjugate inhibited the cell viability of ovarian adenocarcinoma (SK-OV-3) by 59% and the triple-negative breast cancer cells MDA-MB-231 and MCF-7 by 71% and 77%, respectively, at a concentration of 5 μM after 72 h of incubation. In contrast, Dox inhibited the proliferation of SK-OV-3, MDA-MB-231, and MCF-7 by 35%, 63%, and 57%, respectively. Furthermore, [(WR)8WKβA]-Dox conjugate (5 µM) inhibited the cell viability of Dox-resistant cells (MES-SA/MX2) by 92%, while the viability of cells incubated with free Dox was only 15% at 5 μM. Confocal microscopy images confirmed the ability of both Dox conjugate and the physical mixture of the peptide with the drug to deliver Dox through an endocytosis-independent pathway, as the uptake was not inhibited in the presence of endocytosis inhibitors. The stability of Dox conjugate was observed at different time intervals using analytical HPLC when the conjugate was incubated with 25% human serum. Half-life (t1/2) for [(WR)8WKβA]-Dox conjugate was (∼6 h), and more than 80% of the conjugate was degraded at 12 h. The release of free Dox was assessed intracellularly using the CCRF-CEM cell line. The experiment demonstrated that approximately 100% of free Dox was released from the conjugate intracellularly within 72 h. These data confirm the ability of the cyclic cell-penetrating peptide containing tryptophan and arginine residues as an efficient tool for delivery of Dox and for overcoming resistance to it.

Systematic Screening of Penetratin's Protein Targets by Yeast Proteome Microarrays

Cell-penetrating peptides (CPPs) have distinct properties to translocate across cell envelope. The key property of CPPs to translocation with attached molecules has been utilized as vehicles for the delivery of several potential drug candidates that illustrate the significant effect in in-vitro experiment but fail in in-vivo experiment due to selectively permeable nature of cell envelop. Penetratin, a well-known CPP identified from the third α-helix of Antennapedia homeodomain of Drosophila, has been widely used and studied for the delivery of bioactive molecules to treat cancers, stroke, and infections caused by pathogenic organisms. Few studies have demonstrated that penetratin directly possesses antimicrobial activities against bacterial and fungal pathogens; however, the mechanism is unknown. In this study, we have utilized the power of high-throughput Saccharomyces cerevisiae proteome microarrays to screen all the potential protein targets of penetratin. Saccharomyces cerevisiae proteome microarrays assays of penetratin followed by statistical analysis depicted 123 Saccharomyces cerevisiae proteins as the protein targets of penetratin out of ~5800 Saccharomyces cerevisiae proteins. To understand the target patterns of penetratin, enrichment analyses were conducted using 123 protein targets. In biological process: ribonucleoprotein complex biogenesis, nucleic acid metabolic process, actin filament-based process, transcription, DNA-templated, and negative regulation of gene expression are a few significantly enriched terms. Cytoplasm, nucleus, and cell-organelles are enriched terms for cellular component. Protein-protein interactions network depicted ribonucleoprotein complex biogenesis, cortical cytoskeleton, and histone binding, which represent the major enriched terms for the 123 protein targets of penetratin. We also compared the protein targets of penetratin and intracellular protein targets of antifungal AMPs (Lfcin B, Histatin-5, and Sub-5). The comparison results showed few unique proteins between penetratin and AMPs. Nucleic acid metabolic process and cellular component disassembly were the common enrichment terms for penetratin and three AMPs. Penetratin shows unique enrichment items that are related to DNA biological process. Moreover, motif enrichment analysis depicted different enriched motifs in the protein targets of penetratin, LfcinB, Histatin-5, and Sub-5.

Development of Cu(ii)-specific peptide shuttles capable of preventing Cu–amyloid beta toxicity and importing bioavailable Cu into cells

Copper (Cu) in its ionic forms is an essential element for mammals and its homeostasis is tightly controlled. Accordingly, Cu-dyshomeostasis can be lethal as is the case in the well-established genetic Wilson's and Menkes diseases. In Alzheimer's disease (AD), Cu-accumulation occurs in amyloid plaques, where it is bound to the amyloid-beta peptide (Aβ). In vitro, Cu–Aβ is competent to catalyze the production of reactive oxygen species (ROS) in the presence of ascorbate under aerobic conditions, and hence Cu–Aβ is believed to contribute to the oxidative stress in AD. Several molecules that can recover extracellular Cu from Aβ and transport it back into cells with beneficial effects in cell culture and transgenic AD models were identified. However, all the Cu-shuttles currently available are not satisfactory due to various potential limitations including ion selectivity and toxicity. Hence, we designed a novel peptide-based Cu shuttle with the following properties: (i) it contains a Cu(ii)-binding motif that is very selective to Cu(ii) over all other essential metal ions; (ii) it is tagged with a fluorophore sensitive to Cu(ii)-binding and release; (iii) it is made of a peptide platform, which is very versatile to add new functions. The work presented here reports on the characterization of AKH-αR5W4^NBD, which is able to transport Cu ions selectively into PC12 cells and the imported Cu appeared bioavailable, likely via reductive release induced by glutathione. Moreover, AKH-αR5W4^NBD was able to withdraw Cu from the Aβ_1–16 peptide and consequently inhibited the Cu-Aβ based reactive oxygen species production and related cell toxicity. Hence, AKH-αR5W4^NBD could be a valuable new tool for Cu-transport into cells and suitable for mechanistic studies in cell culture, with potential applications in restoring Cu-homeostasis in Cu-related diseases such as AD.

The synthetic peptide AKH-αR5W4^NBD was designed as a shuttle to counteract copper imbalance in Alzheimer’s disease. In vitro, this shuttle is able to abstract Cu(ii) selectively from amyloid-β and transport it into cells in a bioavailable form.

Role of karyopherin nuclear transport receptors in nuclear transport by nuclear trafficking peptide

Nuclear trafficking peptide (NTP), a cell-penetrating peptide (CPP) composed of 10 amino acids (aa) (RIFIHFRIGC), has potent nuclear trafficking activity. Recently, we established a protein-based cell engineering system by using NTP, but it remained elusive how NTP functions as a CPP with nuclear orientation. In the present study, we identified importin subunit β1 (IMB1) and transportin 1 (TNPO1) as cellular proteins underlying the activity of NTP. These karyopherin nuclear transport receptors were identified as candidate molecules by liquid chromatography/mass spectrometry analysis, and downregulation of each protein by small interfering RNA significantly reduced NTP activity (P < 0.01). Biochemical analyses revealed that NTP bound directly to both molecules, and the forced expression of an IMB1 fragment (296-516 aa) or TNPO1 fragment (1-297 aa), which both contain binding sites to NTP, reduced nuclear NTP-green fluorescent protein (GFP) levels when it was added to cell culture medium. NTP is derived from viral protein R (Vpr) of human immunodeficiency virus-1, and Vpr enters the nucleus and exerts pleiotropic functions. Notably, Vpr bound directly to IMB1 and TNPO1, and its function was significantly impaired by the forced expression of the 296-516-aa fragment of IMB1 and 1-297-aa fragment of TNPO1. Interestingly, NTP completely blocked the physical association of Vpr with IMB1 and TNPO1. Although the nuclear localization mechanism of Vpr remains unknown, our data suggest that NTP functions as a novel nuclear localization signal of Vpr.

The impact of phosphatidylserine exposure on cancer cell membranes on the activity of the anticancer peptide HB43

HB43 (FAKLLAKLAKKLL) is a synthetic peptide active against cell lines derived from breast, colon, melanoma, lung, prostate, and cervical cancers. Despite its remarkable spectrum of activity, the mechanism of action at the molecular level has never been investigated, preventing further optimization of its selectivity. The alternation of charged and hydrophobic residues suggests amphipathicity, but the formation of alpha-helical structure seems discouraged by its short length and the large number of positively charged residues. Using different biophysical and in silico approaches we show that HB43 is completely unstructured in solution but assumes alpha-helical conformation in the presence of DPC micelles and liposomes exposing phosphatidylserine (PS) used as mimics of cancer cell membranes. Membrane permeabilization assays demonstrate that the interaction leads to the preferential destabilization of PS-containing vesicles with respect to PC-containing ones, here used as noncancerous cell mimics. ssNMR reveals that HB43 is able to fluidify the internal structure of cancer-cell mimicking liposomes while MD simulations show its internalization in such bilayers. This is achieved by the formation of specific interactions between the lysine side chains and the carboxylate group of phosphatidylserine and/or the phosphate oxygen atoms of targeted phospholipids, which could catalyze the formation of the alpha helix required for internalization. With the aim of better understanding the peptide biocompatibility and the additional antibacterial activity, the interaction with noncancerous cell mimicking liposomes exposing phosphatidylcholine (PC) and bacterial mimicking bilayers exposing phosphatidylglycerol (PG) is also described.

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.

Hybrid Cyclic-Linear Cell-Penetrating Peptides Containing Alternative Positively Charged and Hydrophobic Residues as Molecular Transporters

The cell membrane properties create a significant obstacle in intracellular delivery of cell-impermeable and negatively charged molecules. Herein, we report the synthesis and biological evaluation of a novel series of hybrid cyclic-linear peptides containing alternative positive and hydrophobic amino acids on the ring and side chain [(RW)₅]K(RW)_X (X = 1-5) to compare their molecular transporter efficiency. The peptides were synthesized through Fmoc solid-phase peptide synthesis. In vitro cytotoxicity of the peptides showed that the peptides did not exhibit any significant cytotoxicity at the concentration of 10 μM in human leukemia carcinoma cell line (CCRF-CEM), human ovarian adenocarcinoma cells (SK-OV-3), human epithelial embryonic kidney healthy (HEK-293), and human epithelial mammary gland adenocarcinoma cells (MDA-MB-231) after 3 h incubation. The cellular uptake of a fluorescence-labeled phosphopeptide (F'-GpYEEI) and anti-human immunodeficiency virus (HIV) drugs (lamivudine (F'-3TC), emtricitabine (F'-FTC), Stavudine (F'-d4T)), where F' is carboxyfluorescein, was measured in the presence of the peptides in CCRF-CEM and SK-OV-3 cells. Among all peptides, [(RW)₅K](RW)₅ (10 μM) was the most efficient transporter that improved the cellular uptake of F'-GpYEEI (2 μM) by 18- and 11-fold in CCRF-CEM and SK-OV-3, respectively, compared with F'-GpYEEI alone. Fluorescence-activated cell sorting (FACS) analysis results indicated that the cellular uptake of fluorescence-labeled peptide (F'-[(RW)₅K](RW)₅) was only partially inhibited by chlorpromazine as an endocytosis inhibitor after 3 h incubation in MDA-MB-231 cells. These data suggest the potential of this series of hybrid cyclic-linear peptides as cell-penetrating peptides and molecular transporters.

Delivery of Various Cargos into Cancer Cells and Tissues via Cell-Penetrating Peptides: A Review of the Last Decade

Cell-penetrating peptides (CPPs), also known as protein transduction domains, are a class of diverse amino acid sequences with the ability to cross cellular membranes. CPPs can deliver several bioactive cargos, including proteins, peptides, nucleic acids and chemotherapeutics, into cells. Ever since their discovery, synthetic and natural CPPs have been utilized in therapeutics delivery, gene editing and cell imaging in fundamental research and clinical experiments. Over the years, CPPs have gained significant attention due to their low cytotoxicity and high transduction efficacy. In the last decade, multiple investigations demonstrated the potential of CPPs as carriers for the delivery of therapeutics to treat various types of cancer. Besides their remarkable efficacy owing to fast and efficient delivery, a crucial benefit of CPP-based cancer treatments is delivering anticancer agents selectively, rather than mediating toxicities toward normal tissues. To obtain a higher therapeutic index and to improve cell and tissue selectivity, CPP-cargo constructions can also be complexed with other agents such as nanocarriers and liposomes to obtain encouraging outcomes. This review summarizes various types of CPPs conjugated to anticancer cargos. Furthermore, we present a brief history of CPP utilization as delivery systems for anticancer agents in the last decade and evaluate several reports on the applications of CPPs in basic research and preclinical studies.

Combination of cell-penetrating peptides with nanomaterials for the potential therapeutics of central nervous system disorders: a review

Although nanomedicine have greatly developed and human life span has been extended, we have witnessed the soared incidence of central nervous system (CNS) diseases including neurodegenerative diseases (Alzheimer's disease, Parkinson's disease), ischemic stroke, and brain tumors, which have severely damaged the quality of life and greatly increased the economic and social burdens. Moreover, partial small molecule drugs and almost all large molecule drugs (such as recombinant protein, therapeutic antibody, and nucleic acid) cannot cross the blood-brain barrier. Therefore, it is especially important to develop a drug delivery system that can effectively deliver therapeutic drugs to the central nervous system for the treatment of central nervous system diseases. Cell penetrating peptides (CPPs) provide a potential strategy for the transport of macromolecules through the blood-brain barrier. This study analyzed and summarized the progress of CPPs in CNS diseases from three aspects: CPPs, the conjugates of CPPs and drug, and CPPs modified nanoparticles to provide scientific basis for the application of CPPs for CNS diseases.

Prospect of cell penetrating peptides in stem cell tracking

Stem cell therapy has shown great efficacy in many diseases. However, the treatment mechanism is still unclear, which is a big obstacle for promoting clinical research. Therefore, it is particularly important to track transplanted stem cells in vivo, find out the distribution and condition of the stem cells, and furthermore reveal the treatment mechanism. Many tracking methods have been developed, including magnetic resonance imaging (MRI), fluorescence imaging, and ultrasound imaging (UI). Among them, MRI and UI techniques have been used in clinical. In stem cell tracking, a major drawback of these technologies is that the imaging signal is not strong enough, mainly due to the low cell penetration efficiency of imaging particles. Cell penetrating peptides (CPPs) have been widely used for cargo delivery due to its high efficacy, good safety properties, and wide delivery of various cargoes. However, there are few reports on the application of CPPs in current stem cell tracking methods. In this review, we systematically introduced the mechanism of CPPs into cell membranes and their advantages in stem cell tracking, discussed the clinical applications and limitations of CPPs, and finally we summarized several commonly used CPPs and their specific applications in stem cell tracking. Although it is not an innovation of tracer materials, CPPs as a powerful tool have broad prospects in stem cell tracking.

Treatment of Melanoma by Nano-conjugate-Delivered Wee1 siRNA

Small interfering RNA (siRNA)-based drugs have shown tremendous potential to date in cancer gene therapy. Despite the considerable efforts in siRNA design and manufacturing, unsatisfactory delivery systems persist as a limitation for the application of siRNA-based drugs. In this work, the cholesterol, cell-penetrating peptide conjugate cRGD (R8-cRGD), and polyethylene glycol (PEG) were introduced into low-molecular-weight polyethyleneimine (LMW PEI) to form cRGD-R9-cholesterol-PEI-PEG (RRCPP) nanoparticles with specific targeting and highly penetrating abilities. The enhanced siRNA uptake efficiency of the RRCPP delivery system benefited from R8-cRGD modification. Wee1 is an oncogenic nuclear kinase that can regulate the cell cycle as a crucial G2/M checkpoint. Overexpression of Wee1 in melanoma may lead to a poor prognosis. In the present study, RRCPP nanoparticles were designed for Wee1 siRNA delivery to form an RRCPP/siWee1 complex, which significantly silenced the expression of the WEE1 gene (>60% inhibition) and induced B16 tumor cell apoptosis by abrogating the G2M checkpoint and DNA damage in vitro. Furthermore, the RRCPP/siWee1 complex suppressed B16 tumor growth in a subcutaneous xenograft model (nearly 85% inhibition rate) and lung metastasis (nearly 66% inhibition rate) with ideal in vivo safety. Briefly, our results support the validity of RRCPP as a potential Wee1 siRNA carrier for melanoma gene therapy.

Oral delivery of proteins and peptides: Challenges, status quo and future perspectives

Proteins and peptides (PPs) have gradually become more attractive therapeutic molecules than small molecular drugs due to their high selectivity and efficacy, but fewer side effects. Owing to the poor stability and limited permeability through gastrointestinal (GI) tract and epithelia, the therapeutic PPs are usually administered by parenteral route. Given the big demand for oral administration in clinical use, a variety of researches focused on developing new technologies to overcome GI barriers of PPs, such as enteric coating, enzyme inhibitors, permeation enhancers, nanoparticles, as well as intestinal microdevices. Some new technologies have been developed under clinical trials and even on the market. This review summarizes the history, the physiological barriers and the overcoming approaches, current clinical and preclinical technologies, and future prospects of oral delivery of PPs.

Learning embedding features based on multi-sense-scaled attention architecture to improve the predictive performance of anticancer peptides

MOTIVATION

Anticancer peptides (ACPs) have recently emerged as effective anticancer drugs in cancer therapy. Machine-learning-based predictors have been developed to identify ACPs and achieve satisfactory performance. However, existing methods suffer from experience-based feature engineering, which not only restricts the representation ability of the models to a certain extent but also lacks adaptivity for different data, limiting the further improvement of the predictive performance and impacting the robustness of the predictive models. To alleviate the above problems, we propose a novel deep-learning-based predictor named ACPred-LAF, in which we propose a novel multi-sense and multi-scaled embedding algorithm to automatically learn and extract context sequential characteristics of ACPs.

RESULTS

Through the feature comparative analysis, we demonstrate that our learnable and self-adaptive embedding features are better than hand-crafted features in capturing discriminative information, which can effectively benefit the performance improvement for ACP prediction. In addition, benchmarking comparison results demonstrate that our ACPred-LAF outperforms the state-of-the-art methods both on existing benchmark datasets and our newly constructed dataset. Furthermore, we also prove and validate the robustness of the model via the data interference experiment. To avoid potential evaluation bias, here we construct a new ACP benchmark dataset named ACP-Mixed by integrating existing datasets. We expect our newly constructed dataset to be a golden standard benchmark dataset in this field. To facilitate the use of our model, we develop a web server as the implementation of ACPred-LAF.

AVAILABILITY

Our proposed ACPred-LAF, newly constructed benchmark dataset ACP-Mixed are open source collaborative initiatives available in the GitHub repository (https://github.com/TearsWaiting/ACPred-LAF). Besides, a webserver as the implementation of ACPred-LAF that can be accessed via: http://server.malab.cn/ACPred-LAF.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

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.

Identification of a new cell-penetrating peptide derived from the african swine fever virus CD2v protein

The African swine fever virus (ASFV) is a huge and complex DNA virus that can lead to the acute death of pigs and cause huge losses to the global swine industry. The CD2v protein is a transmembrane protein encoded by the ASFV’s EP402R gene, which can effectively inhibit the bystander lymphocyte proliferation in response to mitogens and mediate the absorption of red blood cells to ASFV-infected cells. The CD2v protein contains repetitive amino acid sequences ([KPCPPP]₃ labeled as RAAS), which is reported as a genetic marker and an epitope. However, the specific biological function of the RAAS is unknown. Here, we have found that the truncated CD2v protein with RAAS can enter Chinese hamster ovary cells, but the truncated CD2v protein without RAAS cannot enter the cells. Also, the RAAS can carry the macromolecular protein EGFP to enter various cells through multiple endocytic processes that are dependent on time, concentration, and location. Besides, the RAAS enter the cells via the macropinocytosis or the clathrin-mediated endocytosis. These results indicate that the RAAS can function as a cell-penetrating peptide that provides a new insight for ASFV research and has potential application value as a tool for drug delivery.

In Vitro and In Vivo Cell Uptake of a Cell-Penetrating Peptide Conjugated with Fluorescent Dyes Having Different Chemical Properties

Simple Summary

In fluorescence imaging employing a targeting strategy, fluorescent dyes conjugated with ligands may alter the pharmacokinetics of the conjugates. The aim of this study was to investigate whether in vitro and in vivo cell uptake are affected when fluorescent dyes with different chemical properties are conjugated with a ligand. The results show that attention should be paid to the chemical properties of fluorescent dyes in designing fluorescent imaging agents.

Abstract

In molecular imaging, a targeting strategy with ligands is widely used because specificity can be significantly improved. In fluorescence imaging based on a targeting strategy, the fluorescent dyes conjugated with ligands may affect the targeting efficiency depending on the chemical properties. Herein, we used a cell-penetrating peptide (CPP) as a ligand with a variety of fluorescent cyanine dye. We investigated in vitro and in vivo cell uptake of the dye-CPP conjugates when cyanine dyes with differing charge and hydrophilicity/lipophilicity were used. The results showed that the conjugates with positively charged and lipophilic cyanine dyes accumulated in cancer cells in vitro, but there was almost no accumulation in tumors in vivo. On the other hand, the conjugates with negatively charged and hydrophilic cyanine dyes did not accumulate in cancer cells in vitro, but fluorescence was observed in tumors in vivo. These results show that there are some cases in which the cell uptake of the dye-peptide conjugates may differ significantly between in vitro and in vivo experiments due to the chemical properties of the fluorescent dyes. This suggests that attention should be paid to the chemical properties of fluorescent dyes in fluorescence imaging based on a targeting strategy.

Synthesis and Characterization of Mannosylated Formulations to Deliver a Minicircle DNA Vaccine

DNA vaccines still represent an emergent area of research, giving rise to continuous progress towards several biomedicine demands. The formulation of delivery systems to specifically target mannose receptors, which are overexpressed on antigen presenting cells (APCs), is considered a suitable strategy to improve the DNA vaccine immunogenicity. The present study developed binary and ternary carriers, based on polyethylenimine (PEI), octa-arginine peptide (R8), and mannose ligands, to specifically deliver a minicircle DNA (mcDNA) vaccine to APCs. Systems were prepared at various nitrogen to phosphate group (N/P) ratios and characterized in terms of their morphology, size, surface charge, and complexation capacity. In vitro studies were conducted to assess the biocompatibility, cell internalization ability, and gene expression of formulated carriers. The high charge density and condensing capacity of both PEI and R8 enhance the interaction with the mcDNA, leading to the formation of smaller particles. The addition of PEI polymer to the R8-mannose/mcDNA binary system reduces the size and increases the zeta potential and system stability. Confocal microscopy studies confirmed intracellular localization of targeting systems, resulting in sustained mcDNA uptake. Furthermore, the efficiency of in vitro transfection can be influenced by the presence of R8-mannose, with great implications for gene expression. R8-mannose/PEI/mcDNA ternary systems can be considered valuable tools to instigate further research, aiming for advances in the DNA vaccine field.

Peripheral Administration of a Cell-Penetrating MOTS-c Analogue Enhances Memory and Attenuates Aβ1-42- or LPS-Induced Memory Impairment through Inhibiting Neuroinflammation

MOTS-c is a 16-amino acid mitochondrial derivative peptide reported to be involved in regulating insulin and metabolic homeostasis via the AMP activated protein kinase (AMPK). AMPK agonist AICAR has been reported to improve cognition. Previous reports also pointed out that MOTS-c may be effective as a therapeutic option toward the prevention of the aging processes. Therefore, we investigated the roles of MOTS-c in the memory recognition process. The results showed that central MOTS-c not only enhanced object and location recognition memory formation and consolidation but also ameliorated the memory deficit induced by Aβ_1-42 or LPS. The memory-ameliorating effects of MOTS-c could be blocked by AMPK inhibitor dorsomorphin. Moreover, MOTS-c treatment significantly increased the phosphorylation of AMPK but not ERK, JNK, and p38 in the hippocampus. The underlying mechanism of MOTS-c neuroprotection may involve inhibiting the activation of astrocytes and microglia and production of proinflammatory cytokines. In addition, we found that peripheral administration of MOTS-c does not cross the blood-brain barrier (BBB) and plays an effect. In order to improve the brain intake of MOTS-c, we screen out (PRR)₅, a cell penetrating peptides, as a carrier for MOTS-c into the brain. Then in the NOR task, intranasal or intravenous MP (cell-penetrating MOTS-c analogue) showed good memory performance on memory formation, memory consolidation, and memory impairment. Near-infrared fluorescent experiments showed the real-time biodistribution in brain after intranasal or intravenous infusion of MP. These results suggested that MOTS-c might be a new potential target for treatment of cognitive decline in AD.

Lipid Membrane Interaction of Peptide/DNA Complexes Designed for Gene Delivery

Among gene delivery systems, peptide-based gene carriers have received significant attention because of their selectivity, biocompatibility, and biodegradability. Since cellular membranes function as a barrier toward exogenous molecules, cell-penetrating peptides (CPPs), which are usually cationic and/or amphiphilic, can serve as efficient carriers to deliver cargo into the cytosol. Here, we examined the interactions of carrier peptides and their DNA complexes with lipid membranes using a quartz crystal microbalance (QCM) and high-speed atomic force microscopy (HS-AFM). The carrier peptides are a 12-residue partial presequence of yeast cytochrome c oxidase subunit IV (Cytcox) and BP100, which are a mitochondria-targeting signal peptide and a CPP, respectively. QCM data showed that BP100 has a higher binding affinity than Cytcox to both plasma membrane- and mitochondrial membrane-mimicking lipid bilayers. The DNA complexes with either Cytcox or BP100 exhibited the same tendency. Furthermore, HS-AFM data demonstrated that the DNA complexes of either peptide can disrupt the lipid membranes, forming larger pores in the case of Cytcox. Our results suggest that the binding affinity of the peptide/DNA complex to the plasma membrane is more critical than its membrane disruption ability in enhancing the cellular uptake of DNA.

Photoinduced Endosomal Escape Mechanism: A View from Photochemical Internalization Mediated by CPP-Photosensitizer Conjugates

Endosomal escape in cell-penetrating peptide (CPP)-based drug/macromolecule delivery systems is frequently insufficient. The CPP-fused molecules tend to remain trapped inside endosomes and end up being degraded rather than delivered into the cytosol. One of the methods for endosomal escape of CPP-fused molecules is photochemical internalization (PCI), which is based on the use of light and a photosensitizer and relies on photoinduced endosomal membrane destabilization to release the cargo molecule. Currently, it remains unclear how this delivery strategy behaves after photostimulation. Recent findings, including our studies using CPP-cargo-photosensitizer conjugates, have shed light on the photoinduced endosomal escape mechanism. In this review, we discuss the structural design of CPP-photosensitizer and CPP-cargo-photosensitizer conjugates, and the PCI mechanism underlying their application.

Site-Selective Functionalization of Methionine Residues via Photoredox Catalysis

Bioconjugation technologies have revolutionized the practice of biology and medicine by allowing access to novel biomolecular scaffolds. New methods for residue-selective bioconjugation are highly sought to expand the toolbox for a variety of bioconjugation applications. Herein we report a site-selective methionine bioconjugation protocol that uses photoexcited lumiflavin to generate open-shell intermediates. This reduction-potential-gated strategy enables access to residues unavailable with traditional nucleophilicity-based conjugation methods. To demonstrate the versatility and robustness of this new protocol, we have modified various proteins and further utilized this functional handle to append diverse biological payloads.

Mitochondria-Targeted Antioxidants: A Step towards Disease Treatment

Mitochondria are the main organelles that produce adenosine 5′-triphosphate (ATP) and reactive oxygen species (ROS) in eukaryotic cells and meanwhile susceptible to oxidative damage. The irreversible oxidative damage in mitochondria has been implicated in various human diseases. Increasing evidence indicates the therapeutic potential of mitochondria-targeted antioxidants (MTAs) for oxidative damage-associated diseases. In this article, we introduce the advantageous properties of MTAs compared with the conventional (nontargeted) ones, review different mitochondria-targeted delivery systems and antioxidants, and summarize their experimental results for various disease treatments in different animal models and clinical trials. The combined evidence demonstrates that mitochondrial redox homeostasis is a potential target for disease treatment. Meanwhile, the limitations and prospects for exploiting MTAs are discussed, which might pave ways for further trial design and drug development.